Generic SCSI target mid-level for Linux (SCST)
==============================================

SCST is designed to provide unified, consistent interface between SCSI
target drivers and Linux kernel and simplify target drivers development
as much as possible. Detail description of SCST's features and internals
could be found on its Internet page http://scst.sourceforge.net.

SCST supports the following I/O modes:

 * Pass-through mode with one to many relationship, i.e. when multiple
   initiators can connect to the exported pass-through devices, for
   the following SCSI devices types: disks (type 0), tapes (type 1),
   processors (type 3), CDROMs (type 5), MO disks (type 7), medium
   changers (type 8) and RAID controllers (type 0xC).

 * FILEIO mode, which allows to use files on file systems or block
   devices as virtual remotely available SCSI disks or CDROMs with
   benefits of the Linux page cache.

 * BLOCKIO mode, which performs direct block IO with a block device,
   bypassing page-cache for all operations. This mode works ideally with
   high-end storage HBAs and for applications that either do not need
   caching between application and disk or need the large block
   throughput.

 * User space mode using scst_user device handler, which allows to
   implement in the user space high performance virtual SCSI
   devices. Comparing with fully in-kernel dev handlers this mode has
   very low overhead (few %%).

 * "Performance" device handlers, which provide in pseudo pass-through
   mode a way for direct performance measurements without overhead of
   actual data transferring from/to underlying SCSI device.

In addition, SCST supports advanced per-initiator access and devices
visibility management, so different initiators could see different set
of devices with different access permissions. See below for details.

Full list of SCST features and comparison with other Linux targets you
can find on http://scst.sourceforge.net/comparison.html.


Installation
------------

To see your devices remotely, you need to add a corresponding LUN for
them (see below how). By default, no local devices are seen remotely.
There must be LUN 0 in each LUNs set (security group), i.e. LUs
numeration must not start from, e.g., 1. Otherwise you will see no
devices on remote initiators and SCST core will write into the kernel
log message: "tgt_dev for LUN 0 not found, command to unexisting LU?"

It is highly recommended to use scstadmin utility for configuring
devices and security groups.

The flow of SCST inialization should be as the following:

1. Load of SCST modules with necessary module parameters, if needed.

2. Configure targets, devices, LUNs, etc. using either scstadmin
(recommended), or the sysfs interface directly as described below.

If you experience problems during modules load or running, check your
kernel logs (or run dmesg command for the few most recent messages).

IMPORTANT: Without loading appropriate device handler, corresponding devices
=========  will be invisible for remote initiators, which could lead to holes
           in the LUN addressing, so automatic device scanning by remote SCSI
           mid-level could not notice the devices. Therefore you will have
	   to add them manually via
	   'echo "- - -" >/sys/class/scsi_host/hostX/scan',
	   where X - is the host number.

IMPORTANT: Working of target and initiator on the same host is
=========  supported, except the following 2 cases: swap over target exported
           device and using a writable mmap over a file from target
	   exported device. The latter means you can't mount a file
	   system over target exported device. In other words, you can
	   freely use any sg, sd, st, etc. devices imported from target
	   on the same host, but you can't mount file systems or put
	   swap on them. This is a limitation of Linux memory/cache
	   manager, because in this case a memory allocation deadlock is
	   possible like: system needs some memory -> it decides to
	   clear some cache -> the cache is needed to be written on a
	   target exported device -> initiator sends request to the
	   target located on the same system -> the target needs memory
	   -> the system needs even more memory -> deadlock.

IMPORTANT: In the current version simultaneous access to local SCSI devices
=========  via standard high-level SCSI drivers (sd, st, sg, etc.) and
           SCST's target drivers is unsupported. Especially it is
	   important for execution via sg and st commands that change
	   the state of devices and their parameters, because that could
	   lead to data corruption. If any such command is done, at
	   least related device handler(s) must be restarted. For block
	   devices READ/WRITE commands using direct disk handler are
	   generally safe.


Usage in failover mode
----------------------

It is recommended to use TEST UNIT READY ("tur") command to check if
SCST target is alive in MPIO configurations.


Device handlers
---------------

Device specific drivers (device handlers) are plugins for SCST, which
help SCST to analyze incoming requests and determine parameters,
specific to various types of devices. If an appropriate device handler
for a SCSI device type isn't loaded, SCST doesn't know how to handle
devices of this type, so they will be invisible for remote initiators
(more precisely, "LUN not supported" sense code will be returned).

In addition to device handlers for real devices, there are VDISK, user
space and "performance" device handlers.

VDISK device handler works over files on file systems and makes from
them virtual remotely available SCSI disks or CDROM's. In addition, it
allows to work directly over a block device, e.g. local IDE or SCSI disk
or ever disk partition, where there is no file systems overhead. Using
block devices comparing to sending SCSI commands directly to SCSI
mid-level via scsi_do_req()/scsi_execute_async() has advantage that data
are transferred via system cache, so it is possible to fully benefit
from caching and read ahead performed by Linux's VM subsystem. The only
disadvantage here that in the FILEIO mode there is superfluous data
copying between the cache and SCST's buffers. This issue is going to be
addressed in one of the future releases. Virtual CDROM's are useful for
remote installation. See below for details how to setup and use VDISK
device handler.

"Performance" device handlers for disks, MO disks and tapes in their
exec() method skip (pretend to execute) all READ and WRITE operations
and thus provide a way for direct link performance measurements without
overhead of actual data transferring from/to underlying SCSI device.

NOTE: Since "perf" device handlers on READ operations don't touch the
====  commands' data buffer, it is returned to remote initiators as it
      was allocated, without even being zeroed. Thus, "perf" device
      handlers impose some security risk, so use them with caution.


Compilation options
-------------------

There are the following compilation options, that could be change using
your favorite kernel configuration Makefile target, e.g. "make xconfig":

 - CONFIG_SCST_DEBUG - if defined, turns on some debugging code,
   including some logging. Makes the driver considerably bigger and slower,
   producing large amount of log data.

 - CONFIG_SCST_TRACING - if defined, turns on ability to log events. Makes the
   driver considerably bigger and leads to some performance loss.

 - CONFIG_SCST_EXTRACHECKS - if defined, adds extra validity checks in
   the various places.

 - CONFIG_SCST_USE_EXPECTED_VALUES - if not defined (default), initiator
   supplied expected data transfer length and direction will be used
   only for verification purposes to return error or warn in case if one
   of them is invalid. Instead, locally decoded from SCSI command values
   will be used. This is necessary for security reasons, because
   otherwise a faulty initiator can crash target by supplying invalid
   value in one of those parameters. This is especially important in
   case of pass-through mode. If CONFIG_SCST_USE_EXPECTED_VALUES is
   defined, initiator supplied expected data transfer length and
   direction will override the locally decoded values. This might be
   necessary if internal SCST commands translation table doesn't contain
   SCSI command, which is used in your environment. You can know that if
   you enable "minor" trace level and have messages like "Unknown
   opcode XX for YY. Should you update scst_scsi_op_table?" in your
   kernel log and your initiator returns an error. Also report those
   messages in the SCST mailing list scst-devel@lists.sourceforge.net.
   Note, that not all SCSI transports support supplying expected values.
   You should try to enable this option if you have a not working with
   SCST pass-through device, for instance, an SATA CDROM.

 - CONFIG_SCST_DEBUG_TM - if defined, turns on task management functions
   debugging, when on LUN 6 some of the commands will be delayed for
   about 60 sec., so making the remote initiator send TM functions, eg
   ABORT TASK and TARGET RESET. Also define
   CONFIG_SCST_TM_DBG_GO_OFFLINE symbol in the Makefile if you want that
   the device eventually become completely unresponsive, or otherwise to
   circle around ABORTs and RESETs code. Needs CONFIG_SCST_DEBUG turned
   on.

 - CONFIG_SCST_STRICT_SERIALIZING - if defined, makes SCST send all commands to
   underlying SCSI device synchronously, one after one. This makes task
   management more reliable, with cost of some performance penalty. This
   is mostly actual for stateful SCSI devices like tapes, where the
   result of command's execution depends from device's settings defined
   by previous commands. Disk and RAID devices are stateless in the most
   cases. The current SCSI core in Linux doesn't allow to abort all
   commands reliably if they sent asynchronously to a stateful device.
   Turned off by default, turn it on if you use stateful device(s) and
   need as much error recovery reliability as possible. As a side effect
   of CONFIG_SCST_STRICT_SERIALIZING, on kernels below 2.6.30 no kernel
   patching is necessary for pass-through device handlers (scst_disk,
   etc.).

 - CONFIG_SCST_TEST_IO_IN_SIRQ - if defined, allows SCST to submit selected
   SCSI commands (TUR and READ/WRITE) from soft-IRQ context (tasklets).
   Enabling it will decrease amount of context switches and slightly
   improve performance. The goal of this option is to be able to measure
   overhead of the context switches. If after enabling this option you
   don't see under load in vmstat output on the target significant
   decrease of amount of context switches, then your target driver
   doesn't submit commands to SCST in IRQ context. For instance,
   iSCSI-SCST doesn't do that, but qla2x00t with
   CONFIG_QLA_TGT_DEBUG_WORK_IN_THREAD disabled - does. This option is
   designed to be used with vdisk NULLIO backend.

   WARNING! Using this option enabled with other backend than vdisk
   NULLIO is unsafe and can lead you to a kernel crash!

 - CONFIG_SCST_STRICT_SECURITY - if defined, makes SCST zero allocated data
   buffers. Undefining it (default) considerably improves performance
   and eases CPU load, but could create a security hole (information
   leakage), so enable it, if you have strict security requirements.

 - CONFIG_SCST_ABORT_CONSIDER_FINISHED_TASKS_AS_NOT_EXISTING - if defined,
   in case when TASK MANAGEMENT function ABORT TASK is trying to abort a
   command, which has already finished, remote initiator, which sent the
   ABORT TASK request, will receive TASK NOT EXIST (or ABORT FAILED)
   response for the ABORT TASK request. This is more logical response,
   since, because the command finished, attempt to abort it failed, but
   some initiators, particularly VMware iSCSI initiator, consider TASK
   NOT EXIST response as if the target got crazy and try to RESET it.
   Then sometimes get crazy itself. So, this option is disabled by
   default.

 - CONFIG_SCST_MEASURE_LATENCY - if defined, provides in "latency" files
   global and per-LUN average commands processing latency statistic. You
   can clear already measured results by writing 0 in each file. Note,
   you need a non-preemptible kernel to have correct results.

 - CONFIG_SCST_DIF_INJECT_CORRUPTED_TAGS - if defined, allows injection
   of corrupted DIF tags according to the Oracle specification. This
   functionality is working only if dif_mode doesn't contain dev_store
   and dif_type is 1.

 - CONFIG_SCST_FORWARD_MODE_PASS_THROUGH - if defined, the pass-through
   subsystem starts working in the forwarding mode, where reservation
   commands processed locally and not passed to the backend SCSI device,
   while COMPARE AND WRITE, EXTENDED COPY and RECEIVE COPY RESULTS
   commands, which normally processed locally by the SCST core, not
   processed locally, but passed to the backend device. Intended to be
   used to implement NON-OPTIMIZED ALUA state together with "forwarding"
   target attribute on the remote node. Disabled by default for safety.

HIGHMEM kernel configurations are fully supported, but not recommended
for performance reasons.


Module parameters
-----------------

Module scst supports the following parameters:

 - scst_threads - allows to set count of SCST's threads. By default it
   is CPU count.

 - scst_max_cmd_mem - sets maximum amount of memory in MB allowed to be
   consumed by the SCST commands for data buffers at any given time. By
   default it is approximately TotalMem/4.


SCST sysfs interface
--------------------

SCST sysfs interface designed to be self descriptive and self
containing. This means that a high level management tool for it can be
written once and automatically support any future sysfs interface
changes (attributes additions or removals, new target drivers and dev
handlers, etc.) without any modifications. Scstadmin is an example of
such management tool.

To implement that an management tool should not be implemented around
drivers and their attributes, but around common rules those drivers and
attributes follow. You can find those rules in SysfsRules file. For
instance, each SCST sysfs file (attribute) can contain in the last line
mark "[key]". It is automatically added to allow scstadmin and other
management tools to see which attributes it should save in the config
file. If you are doing manual attributes manipulations, you can ignore
this mark.

Root of SCST sysfs interface is /sys/kernel/scst_tgt. It has the
following entries:

 - devices - this is a root subdirectory for all SCST devices

 - handlers - this is a root subdirectory for all SCST dev handlers

 - max_tasklet_cmd - specifies how many commands at max can be queued in
   the SCST core simultaneously on a single CPU from all connected
   initiators to allow processing commands on this CPU in soft-IRQ
   context in tasklets. If the count of the commands exceeds this value,
   then all of them will be processed only in SCST threads. This is to
   to prevent possible under heavy load starvation of processes on the
   CPUs serving soft IRQs and in some cases to improve performance by
   more evenly spreading load over available CPUs.

 - sgv - this is a root subdirectory for all SCST SGV caches

 - targets - this is a root subdirectory for all SCST targets

 - setup_id - allows to read and write SCST setup ID. This ID can be
   used in cases, when the same SCST configuration should be installed
   on several targets, but exported from those targets devices should
   have different IDs and SNs. For instance, VDISK dev handler uses this
   ID to generate T10 vendor specific identifier and SN of the devices.

 - poll_us - if polling is desired, sets how many us each SCST thread
   is polling its queue after it became empty in a hope that a new
   command can come. In some cases, polling can significantly increase
   IOPS, especially if low power states on CPU not disabled, because on
   high IOPS polling could be cheaper comparing to spending significant
   time on entering, then exiting CPU low power states + corresponding
   context switches. Disabled, i.e. set to 0, by default.

 - suspend - globally suspends or releases all SCSI activities on all
   devices. Useful for mass management, like adding or deleting LUNs.
   Writing to it value v:

	* v > 0 - suspends activities, but waits no more, than v seconds

	* v = 0 - suspends activities, waits indefinitely

	* V < 0 - releases activities.

   Reading from this attribute returns number of previous suspend
   requests.

 - threads - allows to read and set number of global SCST I/O threads.
   Those threads used with async. dev handlers, for instance, vdisk
   BLOCKIO or NULLIO.

 - trace_cmds - shows current SCST commands up to size of the sysfs
   buffer (4KB)

 - trace_mcmds - shows current SCST management commands up to size of
   the sysfs buffer (4KB)

 - trace_level - allows to enable and disable various tracing
   facilities. See content of this file for help how to use it. See also
   section "Dealing with massive logs" for more info how to make correct
   logs when you enabled trace levels producing a lot of logs data.

 - version - read-only attribute, which allows to see version of
   SCST and enabled optional features.

 - last_sysfs_mgmt_res - read-only attribute returning completion status
   of the last management command. In the sysfs implementation there are
   some problems between internal sysfs and internal SCST locking. To
   avoid them in some cases sysfs calls can return error with errno
   EAGAIN. This doesn't mean the operation failed. It only means that
   the operation queued and not yet completed. To wait for it to
   complete, an management tool should poll this file. If the operation
   hasn't yet completed, it will also return EAGAIN. But after it's
   completed, it will return the result of this operation (0 for success
   or -errno for error). The following two shell functions show how to do
   this:

# Read the SCST sysfs attribute $1. See also scst/README for more information.
scst_sysfs_read() {
    local EAGAIN val

    EAGAIN="Resource temporarily unavailable"
    while true; do
        if val="$(LC_ALL=C cat "$1" 2>&1)"; then
            echo -n "${val%\[key\]}"
            return 0
	elif [ "${val/*: }" != "$EAGAIN" ]; then
            return 1
        fi
        sleep 1
    done
}

# Write $1 into the SCST sysfs attribute $2. See also scst/README for more
# information.
scst_sysfs_write() {
    local EAGAIN status

    EAGAIN="Resource temporarily unavailable"
    if status="$(LC_ALL=C; (echo -n "$1" > "$2") 2>&1)"; then
        return 0
    elif [ "${status/*: }" != "$EAGAIN" ]; then
        return 1
    fi
    scst_sysfs_read /sys/kernel/scst_tgt/last_sysfs_mgmt_res >/dev/null
}

"Devices" subdirectory contains subdirectories for each SCST devices.

Content of each device's subdirectory is dev handler specific. See
documentation for your dev handlers for more info about it as well as
SysfsRules file for more info about common to all dev handlers rules.
SCST dev handlers can have the following common entries:

 - block - allows to temporary block and unblock this device. See below.

 - exported - subdirectory containing links to all LUNs where this
   device was exported.

 - handler - if dev handler determined for this device, this link points
   to it. The handler can be not set for pass-through devices.

 - threads_num - shows and allows to set number of threads in this device's
   threads pool. If 0 - no threads will be created, and global SCST
   threads pool will be used. If <0 - creation of the threads pool is
   prohibited.

 - threads_pool_type - shows and allows to sets threads pool type.
   Possible values: "per_initiator" and "shared". When the value is
   "per_initiator" (default), each session from each initiator will use
   separate dedicated pool of threads. When the value is "shared", all
   sessions from all initiators will share the same per-device pool of
   threads. Valid only if threads_num attribute >0.

 - dump_prs - allows to dump persistent reservations information in the
   kernel log.

 - type - SCSI type of this device

Attribute "block" allows to temporary block and unblock this device.
"Blocking" means that no new commands for this device will go into the
execution stage, but instead will be suspended just before it. The
blocked state is not reached until queue of the corresponding device is
completely drained. You can also call this state "frozen". It is useful
in many cases, like consistent snapshots and graceful shutdown.

On write "block" entry allows the following 3 types of parameters:

 - 1 - block device synchronously, i.e. don't return until this device
 becomes blocked, i.e. until queue of it is not completely drained. Can
 be called as many times as needed.

 - 11 params - block device asynchronously, i.e. return immediately.
 Notification about completing is delivered using SCST_EVENT_EXT_BLOCKING_DONE
 event. "Params" delivered to it as is in "data" payload. Can be
 called as many times as needed. Alternatively, status of blocking could be
 polled by reading this attributes until the second number reaches 0
 (see below).

 - 0 - unblock this device.

Reading from "block" entry returns two numbers separated by space:

1. How many times this device was blocked, i.e. how many times writing
"0" to it is needed to unblock this device.

2. Boolean (0 or 1) if blocking, if any, is done (0) or still pending (1).

See below for more information about other entries of this subdirectory
of the standard SCST dev handlers.

"Handlers" subdirectory contains subdirectories for each SCST dev
handler.

Content of each handler's subdirectory is dev handler specific. See
documentation for your dev handlers for more info about it as well as
SysfsRules file for more info about common to all dev handlers rules.
SCST dev handlers can have the following common entries:

 - mgmt - this entry allows to create virtual devices and their
   attributes (for virtual devices dev handlers) or assign/unassign real
   SCSI devices to/from this dev handler (for pass-through dev
   handlers).

 - trace_level - allows to enable and disable various tracing
   facilities. See content of this file for help how to use it. See also
   section "Dealing with massive logs" for more info how to make correct
   logs when you enabled trace levels producing a lot of logs data.

 - type - SCSI type of devices served by this dev handler.

See below for more information about other entries of this subdirectory
of the standard SCST dev handlers.

"Sgv" subdirectory contains statistic information of SCST SGV caches. It
has the following entries:

 - None, one or more subdirectories for each existing SGV cache.

 - global_stats - file containing global SGV caches statistics.

Each SGV cache's subdirectory has the following item:

 - stats - file containing statistics for this SGV caches.

"Targets" subdirectory contains subdirectories for each SCST target.

Content of each target's subdirectory is target specific. See
documentation for your target for more info about it as well as
SysfsRules file for more info about common to all targets rules.
Every target should have at least the following entries:

 - ini_groups - subdirectory, which contains and allows to define
   initiator-oriented access control information, see below.

 - luns - subdirectory, which contains list of available LUNs in the
   target-oriented access control and allows to define it, see below.

 - sessions - subdirectory containing connected to this target sessions.

 - comment - this attribute can be used to store any human readable info
   to help identify target. For instance, to help identify the target's
   mapping to the corresponding hardware port. It isn't anyhow used by
   SCST.

 - enabled - using this attribute you can enable or disable this target.
   It allows to finish configuring it before it starts accepting new
   connections. 0 by default.

 - addr_method - used LUNs addressing method. Possible values:
   "Peripheral", "Flat" or "LUN". Most initiators work well with
   Peripheral addressing method (default), but some (HP-UX, for instance)
   may require the Flat method or the LUN method (e.g. IBM systems). This
   attribute is also available in the initiators security groups, so you
   can assign the addressing method on per-initiator basis. See also the
   "Logical unit addressing (LUN)" section in SAM-5 for more information.

 - black_hole - if set, all LUNs in the corresponding initiator group,
   default target group in this case, start "swallowing" requests from
   initiators. Possible values are:

    * 0 - disable black hole mode

    * 1 - immediately abort all coming SCSI commands, i.e. all SCSI commands
      are dropped and TM requests return that they completed. It is
      supposed to simulate lost front end responses.

    * 2 - immediately abort all coming SCSI commands and drop all coming TM
      commands. It is supposed to simulate logical target hang, when the
      target stops responding, but on the HW/TCP connection level still
      appears to be online.

    * 3 - immediately abort all coming data transfer SCSI commands, i.e.
      only data transfer SCSI commands are dropped, while commands like
      INQUIRY and TEST UNIT READY pass well. It is supposed to simulate
      flaky front end connectivity, when responses for small commands
      pass well, but big data transfers fail.

    * 4 - immediately abort all coming data transfer SCSI commands and
      drop all coming TM commands. It is supposed to simulate really
      flaky front end connectivity, when TM requests or responses are
      also lost.

   Modes 3 and 4 are the most evil ones, because they are not too well
   handled by many initiator OS'es, including Linux, so they may never
   recover from it.

   Note, dropping TM commands, i.e. not sending response on them,
   implemented not for all target drivers. If it's implemented for your
   particular target driver or not, you can find out by checking traces
   or the target driver's source code.

 - dif_capabilities - if this target supports T10-PI, returns which
   exact DIF capabilities this target supports.

 - dif_checks_failed - if this target supports T10-PI, returns
   statistics how many DIF errors have been detected on the
   corresponding processing stages on this target. It returns 3 rows of
   numbers with 3 numbers in each row: for target driver stage, for SCST
   stage and for dev handler stage. Numbers in each row: how many errors
   detected checking application, reference and guard tags
   correspondingly. Writing to this attribute resets the numbers.

 - cpu_mask - defines CPU affinity mask for threads serving this target.
   For threads serving LUNs it is used only for devices with
   threads_pool_type "per_initiator".

 - io_grouping_type - defines how I/O from sessions to this target are
   grouped together. This I/O grouping is very important for
   performance. By setting this attribute in a right value, you can
   considerably increase performance of your setup. This grouping is
   performed only if you use CFQ I/O scheduler on the target and for
   devices with threads_num >= 0 and, if threads_num > 0, with
   threads_pool_type "per_initiator". Possible values:
   "this_group_only", "never", "auto", or I/O group number >0. When the
   value is "this_group_only" all I/O from all sessions in this target
   will be grouped together. When the value is "never", I/O from
   different sessions will not be grouped together, i.e. all sessions in
   this target will have separate dedicated I/O groups. When the value
   is "auto" (default), all I/O from initiators with the same name
   (iSCSI initiator name, for instance) in all targets will be grouped
   together with a separate dedicated I/O group for each initiator name.
   For iSCSI this mode works well, but other transports usually use
   different initiator names for different sessions, so using such
   transports in MPIO configurations you should either use value
   "this_group_only", or an explicit I/O group number. This attribute is
   also available in the initiators security groups, so you can assign
   the I/O grouping on per-initiator basis. See below for more info how
   to use this attribute.

 - rel_tgt_id - allows to read or write SCSI Relative Target Port
   Identifier attribute. This identifier is used to identify SCSI Target
   Ports by some SCSI commands, mainly by Persistent Reservations
   commands. This identifier must be unique among all SCST targets, but
   for convenience SCST allows disabled targets to have not unique
   rel_tgt_id. In this case SCST will not allow to enable this target
   until rel_tgt_id becomes unique. This attribute initialized unique by
   SCST by default.

 - forwarding - if set this target is forwarding target, i.e. does not check
   any local SCSI events (reservations, etc.). Those event supposed to
   be checked on the another, requester's side.

 - *count*, e.g. read_io_count_kb, - statistics about executed
   commands and transferred data. Those attributes have speaking names
   built from parts:

   1. Data transfer direction

   2. Alignment type: not specified or unaligned (on 4K boundaries)

   3. Type: IO (commands) count or amount of transferred data

   4. For transferred data: measurement units

   For instance, read_unaligned_cmd_count means number of 4K unaligned IOs.

A target driver may have also the following entries:

 - "hw_target" - if the target driver supports both hardware and virtual
    targets (for instance, an FC adapter supporting NPIV, which has
    hardware targets for its physical ports as well as virtual NPIV
    targets), this read only attribute for all hardware targets will
    exist and contain value 1.

Subdirectory "sessions" contains one subdirectory for each connected
session with name equal to name of the connected initiator with the
following entries:

 - initiator_name - contains initiator name

 - force_close - optional write-only attribute, which allows to force
   close this session.

 - active_commands - contains number of active, i.e. not yet or being
   executed, SCSI commands in this session.

 - commands - contains overall number of SCSI commands in this session.

 - dif_checks_failed - if target of this session supports T10-PI, returns
   statistics how many DIF errors have been detected on the
   corresponding processing stages on all DIF-enabled LUNs in this
   session. It returns 3 rows of numbers with 3 numbers in each row: for
   target driver stage, for SCST stage and for dev handler stage.
   Numbers in each row: how many errors detected checking application,
   reference and guard tags correspondingly. Writing to this attribute
   resets the numbers. Similar statistics returned in attribute with the
   same name for each LUN in this session in this LUN's subdirectory, if
   its device configured with dif_type > 0.

 - read_cmd_count - number of READ SCSI commands received since beginning
   or last reset (writing 0 in this attribute)

 - read_io_count_kb - amount of data in KB read by the initiator since
   beginning or last reset (writing 0 in this attribute)

 - write_cmd_count - number of WRITE SCSI commands received since
   beginning or last reset (writing 0 in this attribute)

 - write_io_count_kb - amount of data in KB written by the initiator
   since beginning or last reset (writing 0 in this attribute)

 - bidi_cmd_count - number of BIDI SCSI commands received since
   beginning or last reset (writing 0 in this attribute)

 - bidi_io_count_kb - amount of data in KB transferred by the
   initiator since beginning or last reset (writing 0 in this attribute)

 - none_cmd_count - number of not transferring data SCSI commands
   (e.g. INQUIRY or TEST UNIT READY) received since beginning or last
   reset (writing 0 in this attribute)

 - unknown_cmd_count - number of unknown SCSI commands received since
   beginning or last reset (writing 0 in this attribute)

 - latency - if CONFIG_SCST_MEASURE_LATENCY enabled, contains latency
   statistics for this session.

 - *count*, e.g. read_io_count_kb, - statistics about executed
   commands and transferred data. See above for more details.

 - luns - a link pointing out to the corresponding LUNs set (security
   group) where this session was attached to.

 - One or more "lunX" subdirectories, where 'X' is a number, for each LUN
   this session has (see below).

 - other target driver specific attributes and subdirectories.

See below description of the VDISK's sysfs interface for samples.


Each sessions/<sess>/lun<X> subdirectory contains the following entries:

 - active_commands - contains number of active, i.e. not yet or being
   executed, SCSI commands for lun<X> in session <sess>.

 - thread_pid - contains a single line with all the process identifiers
   (PIDs) of the kernel threads that process SCSI commands intended for
   lun<X> in session <sess>.


Access and devices visibility management (LUN masking)
------------------------------------------------------

Access and devices visibility management allows for an initiator or
group of initiators to see different devices with different LUNs
with necessary access permissions.

SCST supports two modes of access control:

1. Target-oriented. In this mode you define for each target a default
set of LUNs, which are accessible to all initiators, connected to that
target. This is a regular access control mode, which people usually mean
thinking about access control in general. For instance, in IET this is
the only supported mode.

2. Initiator-oriented. In this mode you define which LUNs are accessible
for each initiator. In this mode you should create for each set of one
or more initiators, which should access to the same set of devices with
the same LUNs, a separate security group, then add to it devices and
names of allowed initiator(s).

Both modes can be used simultaneously. In this case the
initiator-oriented mode has higher priority, than the target-oriented,
i.e. initiators are at first searched in all defined security groups for
this target and, if none matches, the default target's set of LUNs is
used. This set of LUNs might be empty, then the initiator will not see
any LUNs from the target.

You can at any time find out which set of LUNs each session is assigned
to by looking where link
/sys/kernel/scst_tgt/targets/target_driver/target_name/sessions/initiator_name/luns
points to.

To configure the target-oriented access control SCST provides the
following interface. Each target's sysfs subdirectory
(/sys/kernel/scst_tgt/targets/target_driver/target_name) has "luns"
subdirectory. This subdirectory contains the list of already defined
target-oriented access control LUNs for this target as well as file
"mgmt". This file has the following commands, which you can send to it,
for instance, using "echo" shell command. You can always get a small
help about supported commands by looking inside this file. "Parameters"
are one or more param_name=value pairs separated by ';'.

 - "add H:C:I:L lun [parameters]" - adds a pass-through device with
   host:channel:id:lun with LUN "lun". Optionally, the device could be
   marked as read only by using parameter "read_only". The recommended
   way to find out H:C:I:L numbers is use of lsscsi utility.

 - "replace H:C:I:L lun [parameters]" - replaces by pass-through device
   with host:channel:id:lun existing with LUN "lun" device with
   generation of INQUIRY DATA HAS CHANGED Unit Attention. If the old
   device doesn't exist, this command acts as the "add" command.
   Optionally, the device could be marked as read only by using
   parameter "read_only". The recommended way to find out H:C:I:L
   numbers is use of lsscsi utility.

 - "add VNAME lun [parameters]" - adds a virtual device with name VNAME
   with LUN "lun". Optionally, the device could be marked as read only
   by using parameter "read_only".

 - "replace VNAME lun [parameters]" - replaces by virtual device
   with name VNAME existing with LUN "lun" device with generation of
   INQUIRY DATA HAS CHANGED Unit Attention. If the old device doesn't
   exist, this command acts as the "add" command. Optionally, the device
   could be marked as read only by using parameter "read_only".

 - "del lun" - deletes LUN lun

 - "clear" - clears the list of devices

To configure the initiator-oriented access control SCST provides the
following interface. Each target's sysfs subdirectory
(/sys/kernel/scst_tgt/targets/target_driver/target_name) has "ini_groups"
subdirectory. This subdirectory contains the list of already defined
security groups for this target as well as file "mgmt". This file has
the following commands, which you can send to it, for instance, using
"echo" shell command. You can always get a small help about supported
commands by looking inside this file.

 - "create GROUP_NAME" - creates a new security group.

 - "del GROUP_NAME" - deletes a new security group.

Each security group's subdirectory contains 2 subdirectories: initiators
and luns as well as the following attributes: addr_method, cpu_mask and
io_grouping_type, black_hole. See above description of them.

Each "initiators" subdirectory contains list of added to this groups
initiator as well as as well as file "mgmt". This file has the following
commands, which you can send to it, for instance, using "echo" shell
command. You can always get a small help about supported commands by
looking inside this file.

 - "add INITIATOR_NAME" - adds initiator with name INITIATOR_NAME to the
   group.

 - "del INITIATOR_NAME" - deletes initiator with name INITIATOR_NAME
   from the group.

 - "move INITIATOR_NAME DEST_GROUP_NAME" moves initiator with name
   INITIATOR_NAME from the current group to group with name
   DEST_GROUP_NAME.

 - "clear" - deletes all initiators from this group.

For "add" and "del" commands INITIATOR_NAME can be a simple DOS-type
patterns, containing '*' and '?' symbols. '*' means match all any
symbols, '?' means match only any single symbol. For instance,
"blah.xxx" will match "bl?h.*". Additionally, you can use negative sign
'!' to revert the value of the pattern. For instance, "ah.xxx" will
match "!bl?h.*".

Each "luns" subdirectory contains the list of already defined LUNs for
this group as well as file "mgmt". Content of this file as well as list
of available in it commands is fully identical to the "luns"
subdirectory of the target-oriented access control.

Examples:

 - echo "create INI" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/mgmt -
   creates security group INI for target iqn.2006-10.net.vlnb:tgt1.

 - echo "add 2:0:1:0 11" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI/luns/mgmt -
   adds a pass-through device sitting on host 2, channel 0, ID 1, LUN 0
   to group with name INI as LUN 11.

 - echo "add disk1 0" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI/luns/mgmt -
   adds a virtual disk with name disk1 to group with name INI as LUN 0.

 - echo "add 21:*:e0:?b:83:*" >/sys/kernel/scst_tgt/targets/21:00:00:a0:8c:54:52:12/ini_groups/INI/initiators/mgmt -
   adds a pattern to group with name INI to Fibre Channel target with
   WWN 21:00:00:a0:8c:54:52:12, which matches WWNs of Fibre Channel
   initiator ports.

Consider you need to have an iSCSI target with name
"iqn.2007-05.com.example:storage.disk1.sys1.xyz", which should export
virtual device "dev1" with LUN 0 and virtual device "dev2" with LUN 1,
but initiator with name
"iqn.2007-05.com.example:storage.disk1.spec_ini.xyz" should see only
virtual device "dev2" read only with LUN 0. To achieve that you should
do the following commands:

# echo "iqn.2007-05.com.example:storage.disk1.sys1.xyz" >/sys/kernel/scst_tgt/targets/iscsi/mgmt
# echo "add dev1 0" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/luns/mgmt
# echo "add dev2 1" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/luns/mgmt
# echo "create SPEC_INI" >/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/ini_groups/mgmt
# echo "add dev2 0 read_only=1" \
	>/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/ini_groups/SPEC_INI/luns/mgmt
# echo "iqn.2007-05.com.example:storage.disk1.spec_ini.xyz" \
	>/sys/kernel/scst_tgt/targets/iscsi/iqn.2007-05.com.example:storage.disk1.sys1.xyz/ini_groups/SPEC_INI/initiators/mgmt

For Fibre Channel or SAS in the above example you should use target's
and initiator ports WWNs instead of iSCSI names.

It is highly recommended to use scstadmin utility instead of described
in this section low level interface.

IMPORTANT
=========

There must be LUN 0 in each set of LUNs, i.e. LUs numeration must not
start from, e.g., 1. Otherwise you will see no devices on remote
initiators and SCST core will write into the kernel log message: "tgt_dev
for LUN 0 not found, command to unexisting LU?"

IMPORTANT
=========

All the access control must be fully configured BEFORE the corresponding
target is enabled. When you enable a target, it will immediately start
accepting new connections, hence creating new sessions, and those new
sessions will be assigned to security groups according to the
*currently* configured access control settings. For instance, to
the default target's set of LUNs, instead of "HOST004" group as you may
need, because "HOST004" doesn't exist yet. So, you must configure all
the security groups before new connections from the initiators are
created, i.e. before the target enabled.


VDISK device handler
--------------------

VDISK has 4 built-in dev handlers: vdisk_fileio, vdisk_blockio,
vdisk_nullio and vcdrom. Roots of their sysfs interface are
/sys/kernel/scst_tgt/handlers/handler_name, e.g. for vdisk_fileio:
/sys/kernel/scst_tgt/handlers/vdisk_fileio. Each root has the following
entries:

 - None, one or more links to devices with name equal to names
   of the corresponding devices.

 - trace_level - allows to enable and disable various tracing
   facilities. See content of this file for help how to use it. See also
   section "Dealing with massive logs" for more info how to make correct
   logs when you enabled trace levels producing a lot of logs data.

 - mgmt - main management entry, which allows to add/delete VDISK
   devices with the corresponding type.

The "mgmt" file has the following commands, which you can send to it,
for instance, using "echo" shell command. You can always get a small
help about supported commands by looking inside this file. "Parameters"
are one or more param_name=value pairs separated by ';'.

 - echo "add_device device_name [parameters]" - adds a virtual device
   with name device_name and specified parameters (see below)

 - echo "del_device device_name" - deletes a virtual device with name
   device_name.

Handler vdisk_fileio provides FILEIO mode to create virtual devices.
This mode uses as backend files and accesses to them using regular
read()/write() file calls. This allows to use full power of Linux page
cache. The following parameters possible for vdisk_fileio:

 - filename - specifies path and file name of the backend file. The path
   must be absolute.

 - blocksize - specifies block size used by this virtual device. The
   block size must be power of 2 and >= 512 bytes. Default is 512.

 - write_through - disables write back caching. Note, this option
   has sense only if you also *manually* disable write-back cache in
   *all* your backstorage devices and make sure it's actually disabled,
   since many devices are known to lie about this mode to get better
   benchmark results. Default is 0.

 - read_only - read only. Default is 0.

 - o_direct - disables both read and write caching. This mode isn't
   currently fully implemented, you should use user space fileio_tgt
   program in O_DIRECT mode instead (see below).

 - nv_cache - enables "non-volatile cache" mode. In this mode it is
   assumed that the target has a GOOD UPS with ability to cleanly
   shutdown target in case of power failure and it is software/hardware
   bugs free, i.e. all data from the target's cache are guaranteed
   sooner or later to go to the media. Hence all data synchronization
   with media operations, like SYNCHRONIZE_CACHE, are ignored in order
   to bring more performance. Also in this mode target reports to
   initiators that the corresponding device has write-through cache to
   disable all write-back cache workarounds used by initiators. Use with
   extreme caution, since in this mode after a crash of the target
   journaled file systems don't guarantee the consistency after journal
   recovery, therefore manual fsck MUST be ran. Note, that since usually
   the journal barrier protection (see "IMPORTANT" note below) turned
   off, enabling NV_CACHE could change nothing from data protection
   point of view, since no data synchronization with media operations
   will go from the initiator. This option overrides "write_through"
   option. Disabled by default.

 - thin_provisioned - enables thin provisioning facility, when remote
   initiators can unmap blocks of storage, if they don't need them
   anymore. Backend storage also must support this facility.

 - tst - allows to specify TST control mode page field. It specifies
   the type of task set in the device. Possible values are: 0 - the
   device maintains one task set for all I_T nexuses and 1 - the device
   maintains separate task sets for each I_T nexus. Default - 1.

 - removable - with this flag set the device is reported to remote
   initiators as removable.

 - rotational - if set, this device reported as rotational. Otherwise,
   it is reported as non-rotational (SSD, etc.)

 - zero_copy - if set, then this device uses zero copy access to the
   page cache. At the moment, only read side zero copy is implemented.

 - dif_mode - specifies which T10-PI, or DIF, mode this device will use.
   See SCSI standards from more info about T10-PI. Available DIF modes
   (can be combined using '|'):

    * tgt - DIF tags are checked on the target hardware, if supported

    * scst - DIF tags are checked inside SCST core

    * dev_check - DIF tags are checked inside backend device. No DIF
      tags storing is required, but optionally possible.

    * dev_store - DIF tags are stored inside backend device on the WRITE
      path and read from it on the READ path. No DIF tags checking is
      required, but optionally possible.

   For instance, if only tgt DIF mode specified, then target driver,
   serving this device, will inside hardware check, then STRIP DIF tags
   from SCSI commands on the WRITE path and generate, then INSERT DIF
   tags into SCSI commands on the READ path, so neither SCST core, nor
   dev handler will see them.

   Similarly, if only scst DIF mode specified, then target driver will
   PASS DIF tags into SCST core, which then check/STRIP/generate/INSERT
   them, so dev handler will not see them.

   If only dev_check DIF mode specified, then both target driver and
   SCST core will PASS DIF tags into the dev handler, which is then
   responsible to check them in the backend hardware. If only dev_store
   specified, then DIF tags will only be stored by the dev handler in
   the backend hardware without checking at any level.

   If all "tgt|scst|dev_check|dev_store" DIF mode specified, then all
   target driver, SCST core and dev handler will check DIF tags, then
   dev handler will store them in the backend hardware.

 - dif_type - specifies which DIF SCSI type this device will use.

 - dif_static_app_tag - specifies fixed (static) DIF application tag for
   this device.

 - dif_filename - specifies full path to filename, where DIF tags will
   be stored.

Handler vdisk_blockio provides BLOCKIO mode to create virtual devices.
This mode performs direct block I/O with a block device, bypassing the
page cache for all operations. This mode works ideally with high-end
storage HBAs and for applications that either do not need caching
between application and disk or need the large block throughput. See
below for more info.

The following parameters possible for vdisk_blockio: filename,
blocksize, nv_cache, read_only, removable, rotational, thin_provisioned,
tst, dif_mode, dif_type, dif_static_app_tag, dif_filename. See
vdisk_fileio above for description of those parameters.

Handler vdisk_nullio provides NULLIO mode to create virtual devices. In
this mode no real I/O is done, but success returned to initiators.
Intended to be used for performance measurements at the same way as
"*_perf" handlers. The following parameters possible for vdisk_nullio:
blocksize, read_only, removable, tst. See vdisk_fileio above for
description of those parameters.

vdisk_nullio devices have the following two additional attributes:

 - dummy - if this flag is set, LUNs corresponding to this device will
   not appear at the initiator side. This is because SCST will set the
   PERIPHERAL QUALIFIER qualifier field to 1 (not connected) and the
   PERIPHERAL DEVICE TYPE to 0x1f (no device) in the INQUIRY response.
   See also SPC-4 for more information. It is designed to be used as a
   "dummy" placeholder on LUN 0, if LUN 0 is not desired.

 - read_zero - if this flag is set, reading from a vdisk_nullio device
   returns a buffer filled with byte 0x00. If this flag is cleared
   (which is the default behavior), the buffer returned to the
   initiator is not cleared. Although this results in slightly faster
   operation this is a security hole since any data that is present in
   kernel memory can be returned to the initiator.

Handler vcdrom allows emulation of a virtual CDROM device using an ISO
file as backend. It has only single parameter: tst.

For example:

echo "add_device disk1 filename=/disk1; blocksize=4096; nv_cache=1" >/sys/kernel/scst_tgt/handlers/vdisk_fileio/mgmt

will create a FILEIO virtual device disk1 with backend file /disk1
with block size 4K and NV_CACHE enabled.

Each vdisk_fileio's device has the following attributes in
/sys/kernel/scst_tgt/devices/device_name:

 - filename - contains path and file name of the backend file.

 - blocksize - contains block size used by this virtual device.

 - write_through - contains status of write back caching of this virtual
   device.

 - sync - writing into this attribute causes the page cache contents to
   be flushed to disk.

 - read_only - contains read only status of this virtual device.

 - o_direct - contains O_DIRECT status of this virtual device.

 - inq_vend_specific - Vendor specific data that will be reported via
   either bytes 36..55 or bytes 96..256 of the INQUIRY response, depending
   on whether this field is <= 20 or > 20 bytes long.

 - nv_cache - contains NV_CACHE status of this virtual device.

 - prod_id - PRODUCT IDENTIFICATION as reported via the INQUIRY response.
   The default value for this field is the SCST device name.

 - prod_rev_lvl - PRODUCT REVISION LEVEL as reported via the INQUIRY
   response. The default value for this field is " 300".

 - scsi_device_name - optional SCSI target device name to which this
   SCST device belongs to (in SCSI terminology all SCST devices called
   Logical Units). See SPC for more info.

 - tst - contains TST field of SCSI Control mode page. See SPC-4 for
   more details about this field.

 - thin_provisioned - contains thin provisioning status of this virtual
   device.

 - removable - contains removable status of this virtual device.

 - rotational - contains rotational status of this virtual device.

 - size_mb - contains size of this virtual device in MB.

 - pr_file_name - Full path of the file or block device in which to store
   persistent reservation information. The default value for this attribute is
   /var/lib/scst/pr/${device_name}. Writing a new value into this sysfs
   attribute is only allowed if the device is not exported. Modifying this
   sysfs attribute causes the persistent reservation state to be reloaded.

 - t10_dev_id - contains and allows to set T10 vendor specific
   identifier for Device Identification VPD page (0x83) of INQUIRY data.
   By default VDISK handler always generates t10_dev_id for every new
   created device at creation time based on the device name and
   scst_vdisk_ID scst_vdisk.ko module parameter (see below).
   Note: some initiators, e.g. VMware's ESXi or MS Hyper-V, only looks
   at the first eight characters of t10_dev_id. You have to make sure
   that these first eight characters are unique or VMware will consider
   these devices as identical.

 - eui64_id - allows to set the EUI-64 based device identifier in the
   SCSI device identification VPD page (83h). This identifier must be 8,
   12 or 16 bytes long and must be specified in hexadecimal format (EUI =
   Extended Unique Identifier). A leading "0x" is allowed but is not
   required. Writing a newline into this attribute discards the EUI-64
   identifier. If neither eui64_id nor naa_id have been set the first
   eight bytes of the t10_dev_id are used as the EUI-64 ID. If naa_id has
   been set but eui64_id has not been set no EUI-64 identifier is
   reported in the SCSI device identification VPD page. If eui64_id has
   been set the value of this attribute is reported as the EUI-64 ID. The
   first three bytes of an EUI-64 ID are a so-called organizationally
   unique identifier (OUI). The remaining bytes may be chosen by the
   organization that owns the OUI. For more information about OUIs, see
   also http://standards.ieee.org/develop/regauth/oui/public.html.

 - naa_id - allows to set the NAA ID in the SCSI INQUIRY response (NAA =
   Network Address Authority). This identifier must be 8 or 16 bytes long
   and must be specified in hex format. A leading "0x" is allowed but is
   not required. Writing a newline into this attribute discards the NAA
   ID. If this ID is set it is reported in the SCSI VPD device
   identification page (83h). More information about NAA identifiers can
   be found in the following documents:
   * ANSI T11 committee, Fibre Channel Framing and Signaling Interface - 4
     (FC-FS-4) rev 0.50, May 2014 (http://www.t11.org/).
   * IETF, RFC 3980 - T11 Network Address Authority (NAA) Naming Format for
     iSCSI Node Names, February 2005 (https://tools.ietf.org/html/rfc3980).

 - t10_vend_id - Contents of the T10 VENDOR IDENTIFICATION field of the
   INQUIRY response. The default value for this field is "SCST_BIO" for
   vdisk_block devices and "SCST_FIO" for vdisk_fileio devices.

 - usn - contains the virtual device's serial number of INQUIRY data. It
   is created at the device creation time based on the device name and
   scst_vdisk_ID scst_vdisk.ko module parameter (see below).

 - type - contains SCSI type of this virtual device.

 - resync_size - write only attribute, which makes vdisk_fileio to
   rescan size of the backend file. It is useful if you changed it, for
   instance, if you resized it.

 - vend_specific_id - Vendor specific ID as reported via the Device
   Identification VPD page (83h). The default value for this attribute
   is the value of the t10_dev_id attribute.

For example:

/sys/kernel/scst_tgt/devices/disk1
|-- block
|-- blocksize
|-- exported
|   |-- export0 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt/luns/0
|   |-- export1 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt/ini_groups/INI/luns/0
|   |-- export2 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt1/luns/0
|   |-- export3 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI1/luns/0
|   |-- export4 -> ../../../targets/iscsi/iqn.2006-10.net.vlnb:tgt1/ini_groups/INI2/luns/0
|-- filename
|-- handler -> ../../handlers/vdisk_fileio
|-- nv_cache
|-- o_direct
|-- read_only
|-- removable
|-- resync_size
|-- rotational
|-- size_mb
|-- t10_dev_id
|-- thin_provisioned
|-- threads_num
|-- threads_pool_type
|-- tst
|-- type
|-- usn
`-- write_through

Each vdisk_blockio's device has the following attributes in
/sys/kernel/scst_tgt/devices/device_name: blocksize, filename, nv_cache,
read_only, removable, resync_size, rotational, size_mb, t10_dev_id,
thin_provisioned, threads_num, threads_pool_type, tst, type, usn. See
above description of those parameters.

Each vdisk_nullio's device has the following attributes in
/sys/kernel/scst_tgt/devices/device_name: blocksize, read_only,
removable, size_mb, t10_dev_id, threads_num, threads_pool_type, type,
tst, usn, dummy. See above description of those parameters.

Each vcdrom's device has the following attributes in
/sys/kernel/scst_tgt/devices/device_name: filename, size_mb,
t10_dev_id, threads_num, threads_pool_type, type, usn, tst. See above
description of those parameters. Exception is filename attribute. For
vcdrom it is writable. Writing to it allows to virtually insert or
change virtual CD media in the virtual CDROM device. For example:

 - echo "/image.iso" >/sys/kernel/scst_tgt/devices/cdrom/filename - will
   insert file /image.iso as virtual media to the virtual CDROM cdrom.

 - echo "" >/sys/kernel/scst_tgt/devices/cdrom/filename - will remove
   "media" from the virtual CDROM cdrom.

Additionally VDISK handler has module parameter "num_threads", which
specifies count of I/O threads for each FILEIO VDISK's or VCDROM device.
If you have a workload, which tends to produce rather random accesses
(e.g. DB-like), you should increase this count to a bigger value, like
32. If you have a rather sequential workload, you should decrease it to
a lower value, like number of CPUs on the target or even 1. Due to some
limitations of Linux I/O subsystem, increasing number of I/O threads too
much leads to sequential performance drop, especially with deadline
scheduler, so decreasing it can improve sequential performance. The
default provides a good compromise between random and sequential
accesses.

You shouldn't be afraid to have too many VDISK I/O threads if you have
many VDISK devices. Kernel threads consume very little amount of
resources (several KBs) and only necessary threads will be used by SCST,
so the threads will not trash your system.

CAUTION: If you partitioned/formatted your device with block size X, *NEVER*
======== ever try to export and then mount it (even accidentally) with another
         block size. Otherwise you can *instantly* damage it pretty
	 badly as well as all your data on it. Messages on initiator
	 like: "attempt to access beyond end of device" is the sign of
	 such damage.

	 Moreover, if you want to compare how well different block sizes
	 work for you, you **MUST** EVERY TIME AFTER CHANGING BLOCK SIZE
	 **COMPLETELY** **WIPE OFF** ALL THE DATA FROM THE DEVICE. In
	 other words, THE **WHOLE** DEVICE **MUST** HAVE ONLY **ZEROS**
	 AS THE DATA AFTER YOU SWITCH TO NEW BLOCK SIZE. Switching block
	 sizes isn't like switching between FILEIO and BLOCKIO, after
	 changing block size all previously written with another block
	 size data MUST BE ERASED. Otherwise you will have a full set of
	 very weird behaviors, because blocks addressing will be
	 changed, but initiators in most cases will not have a
	 possibility to detect that old addresses written on the device
	 in, e.g., partition table, don't refer anymore to what they are
	 intended to refer.

IMPORTANT: Some disk and partition table management utilities don't support
=========  block sizes >512 bytes, therefore make sure that your favorite one
           supports it. Currently only cfdisk is known to work only with
	   512 bytes blocks, other utilities like fdisk on Linux or
	   standard disk manager on Windows are proved to work well with
	   non-512 bytes blocks. Note, if you export a disk file or
	   device with some block size, different from one, with which
	   it was already partitioned, you could get various weird
	   things like utilities hang up or other unexpected behavior.
	   Hence, to be sure, zero the exported file or device before
	   the first access to it from the remote initiator with another
	   block size. On Window initiator make sure you "Set Signature"
	   in the disk manager on the imported from the target drive
	   before doing any other partitioning on it. After you
	   successfully mounted a file system over non-512 bytes block
	   size device, the block size stops matter, any program will
	   work with files on such file system.


Dealing with massive logs
-------------------------

If you want to enable using "trace_level" file logging levels, which
produce a lot of events, like "debug", to not loose logged events you
should also:

  * Increase in .config of your kernel CONFIG_LOG_BUF_SHIFT variable
    to much bigger value, then recompile it. For example, value 25 will
    provide good protection from logging overflow even under high volume
    of logging events. To use it you will need to modify the maximum
    allowed value for CONFIG_LOG_BUF_SHIFT in the corresponding Kconfig
    file to 25 as well.

  * Change in your /etc/syslog.conf or other config file of your favorite
    logging program to store kernel logs in async manner. For example,
    you can add in rsyslog.conf line "kern.info -/var/log/kernel" and
    add "kern.none" in line for /var/log/messages, so the resulting line
    would looks like:

    "*.info;kern.none;mail.none;authpriv.none;cron.none /var/log/messages"


Persistent Reservations
-----------------------

SCST implements Persistent Reservations with full set of capabilities,
including "Persistence Through Power Loss".

The "Persistence Through Power Loss" data are saved in /var/lib/scst/pr
with files with names the same as the names of the corresponding
devices. Also this directory contains backup versions of those files
with suffix ".1". Those backup files are used in case of power or other
failure to prevent Persistent Reservation information from corruption
during update. It is safe to assume that each of those files can be up
to 1KB big.

The Persistent Reservations available on all transports implementing
get_initiator_port_transport_id() callback. Transports not implementing
this callback will act in one of 2 possible scenarios ("all or
nothing"):

1. If a device has such transport connected and doesn't have persistent
reservations, it will refuse Persistent Reservations commands as if it
doesn't support them.

2. If a device has persistent reservations, all initiators newly
connecting via such transports will not see this device. After all
persistent reservations from this device are released, upon reconnect
the initiators will see it.


ALUA Support
------------

SCST supports both implicit and explicit asymmetric logical unit access
(ALUA). ALUA is a feature defined by the ANSI T10 SCSI committee. It
allows a target to tell the initiator which path to use in a multipath
setup plus, in the explicit case, control state of each path via SET
TARGET PORT GROUPS SCSI command. The redundant paths between initiator
and target can be used either for redundancy or for load sharing
purposes. The target can either be a single target system running SCST
with multiple communication interfaces or two target systems each
running SCST and configured in a high availability setup.

In the SPC-4 standard the following concepts are defined related to ALUA:
* Relative target port ID. A number between 1 and 65535 that uniquely
  identifies a target port. These numbers must be unique over the target as
  a whole, even if that target consists of multiple systems each running SCST.
* Target port group asymmetric access state. One of active/optimized,
  active/non-optimized, standby, unavailable, logical block dependent or
  offline. The access state of a port defines which (if any) SCSI commands
  will be processed by the target port.
* Target port preference indicator. This indicator is additional information
  next to the asymmetric access state that is provided by the target to an
  initiator and that may impact the decision taken by the initiator about
  which path that will be chosen.

More detailed information about ALUA can be found in section 5.11.2 of the
ANSI T10 standard called SPC-4.

ALUA support in SCST
....................

SCST allows to define ALUA settings for each unique combination of SCST
device and SCST target. An initiator however queries ALUA settings by
sending an appropriate SCSI command to a specific LUN of an SCST target.
Each such LUN maps uniquely to an SCST device. For hardware SCST target
drivers, e.g. ib_srpt, there is a one-to-one correspondence between SCST
target and SCSI target port. With other SCST targets, e.g. iSCSI-SCST,
by default the only relationship between SCST targets and SCSI target
ports is that all SCST targets defined on a system are visible via all
SCSI target ports. See also the iSCSI-SCST documentation about the
allowed_portal attribute for information about how to associate iSCSI
targets with a single physical interface.

Notes:
- In a H.A. setup it is the responsibility of the user to synchronize ALUA
  information between the individual systems running SCST. There are no
  provisions in SCST to exchange ALUA information automatically between
  individual systems.
- In order to support H.A. setups it is possible to let one SCST system
  report information about target ports present in other SCST systems.
- With SCST, and certainly in a H.A. setup, it is possible to configure ALUA
  such that an initiator receives information that is not standard compliant,
  e.g. setting all target ports in the offline state. It is the responsibility
  of the user to make sure that the information queried by an initiator is
  consistent independent of the LUN and the target port used by the initiator
  to query this information.
- Before building a H.A. setup consisting of two or more SCST systems one
  should evaluate whether it's acceptable that persistent reservation commands,
  SCSI task management commands and MODE SELECT commands will only be processed
  by a single node instead of being processed by all nodes.

Configuring ALUA in SCST
........................

SCST allows to configure the following settings related to ALUA
for each unique combination of SCST target and virtual SCST device
(vdisk_fileio, vdisk_blockio, vcdrom, ...):
* The target port group asymmetric access state. SCST supports all ALUA port
  states except logical block dependent.
* The preference indicator for a target port group.
* The relative target port ID associated with the SCST target.

It is possible to configure the following ALUA-related information via the
sysfs interface of SCST:
* Device groups, where each device group has a name and contains zero or more
  SCST devices. If a device group contains only a single SCST device, the name
  of the group may be identical to the device name. See also
  /sys/kernel/scst_tgt/device_groups/mgmt.
* Which devices are inside a device group. See also
  /sys/kernel/scst_tgt/device_groups/<device group name>/devices/mgmt.
* Target groups, where each target group has a name and contains zero or more
  SCST target names. See also
  /sys/kernel/scst_tgt/device_groups/<device group name>/target_groups/mgmt.
* Target port group identifier. This is a number in the range 0..65535 and is
  called the TARGET PORT GROUP in SPC-4. See also
  /sys/kernel/scst_tgt/device_groups/<device group name>/target_groups/<target
  group name>/group_id.
* Target port group preference indicator. This is a boolean value called the
  PREF bit in SPC-4. See also /sys/kernel/scst_tgt/device_groups/<device group
  name>/target_groups/<target group name>/preferred.
* Target port group state name. One of active, nonoptimized, standby,
  unavailable, offline or transitioning. See also
  /sys/kernel/scst_tgt/device_groups/<device group name>/target_groups/<target
  group name>/state.
* Target group contents - zero or more target names. The target names either
  exist on the local system or on a remote system in a H.A. setup. For target
  names that refer to SCST targets on another system only the relative target
  port identifier matters, not the assigned name. See also
  /sys/kernel/scst_tgt/device_groups/<device group name>/target_groups/<target
  group name>/mgmt.
* Relative target identifier. See also
  /sys/kernel/scst_tgt/device_groups/<device group name>/target_groups/<target
  group name>/<target name>/rel_tgt_id.

The steps involved in configuring ALUA are:
* Identify the SCST devices that will always share the same ALUA settings and
  state. Assign a name to each such group of SCST devices. If a device group
  only contains a single device, the group name may be identical to the device
  name.
* Configure that device group in SCST via sysfs.
* Identify the SCSI target ports that will always share the same ALUA settings
  and state. Assign a name, a group ID and preference indicator to each such
  SCSI target port group.
* Configure the target port group information in SCST via sysfs.
* Identify all SCST targets that can be accessed via a target port group.
* Assign all these SCST target names to the target group via sysfs.
* Assign a relative target port identifier to each target.

As an example, in a H.A. setup with two systems each having one InfiniBand
HCA controlled by the ib_srpt driver and where each system exports two LUNs
the following configuration can be used in scst.conf on both systems:

DEVICE_GROUP dgroup1 {
	DEVICE disk01

	TARGET_GROUP tgroup1 {
		group_id 256
		preferred 1
		state active
		TARGET fe80:0000:0000:0000:0002:c903:00fa:b7e1 {
			rel_tgt_id 1
		}
	}
	TARGET_GROUP tgroup2 {
		group_id 257
		state standby
		TARGET fe80:0000:0000:0000:0002:c903:00fa:b7f2 {
			rel_tgt_id 2
		}
	}
}

DEVICE_GROUP dgroup2 {
	DEVICE disk02

	TARGET_GROUP tgroup1 {
		group_id 258
		state standby
		TARGET fe80:0000:0000:0000:0002:c903:00fa:b7e1 {
			rel_tgt_id 1
		}
	}
	TARGET_GROUP tgroup2 {
		group_id 259
		preferred 1
		state active
		TARGET fe80:0000:0000:0000:0002:c903:00fa:b7f2 {
			rel_tgt_id 2
		}
	}
}

Explicit ALUA
.............

To enable explicit ALUA you need in addition to the above settings set
expl_alua device attribute to 1 (by default it is 0). Also you need to
run stpgd and supply to it path to a script or program, which will
perform actual path state switching on SET TARGET PORT GROUPS command,
for instance, by calling drbdadm. For more information see stpgd README
as well as sample script scst_on_stpg.

DRBD and other replication/failover SW compatibility
....................................................

DRBD as well as other replication/failover SW does not allow to open its
device on the secondary as well as does not allow to perform primary to
secondary transition, if this device is open.

SCST BLOCKIO handler has all the necessary support for such behavior. If
you write new ALUA state in the "state" attribute, SCST BLOCKIO handler
before transition closes the open devices' handles and after transition
reopens them, if the new state is active or nonoptimized.

Thus, the recommended implicit ALUA state change procedure for primary
to secondary transition is:

1. Block all involved SCST devices using "block" sysfs attribute (see
above). Wait until the blocking finished.

2. Change the ALUA state to "transitioning". At this moment all open
file handles will be closed.

3. Perform the DRBD or other replication/failover SW state transition

4. Change the ALUA state to your desired secondary state.

5. Unblock the blocked on step 1 devices.

Optionally, if your initiators support Transitioning ALUA state, for
more responsive behavior the blocked devices can be unblocked
immediately after step (2). However, not all initiators correctly
behave, if they receive ASYMMETRIC STATE TRANSITION sense.

For the secondary to primary transition procedure is similar.

In case of explicit ALUA, SCST automatically performs the necessary
devices blocking around sending SCST_EVENT_STPG_USER_INVOKE event.

Checking the Target Configuration
.................................

One way to verify the ALUA configuration from a Linux initiator is via
the commands provided in the sg3_utils package. The first step is to
verify whether for a certain LUN ALUA has been configured on the target.
This is possible by checking whether the TPGS=1 text appears in the
sg_inq output, where /dev/sdb is a device node created by the ib_srp
initiator:

# sg_inq /dev/sdb
standard INQUIRY:
  PQual=0  Device_type=0  RMB=0  version=0x05  [SPC-3]
  [AERC=0]  [TrmTsk=0]  NormACA=0  HiSUP=1  Resp_data_format=2
  SCCS=0  ACC=0  TPGS=1  3PC=0  Protect=0  BQue=0
  EncServ=0  MultiP=0  [MChngr=0]  [ACKREQQ=0]  Addr16=1
  [RelAdr=0]  WBus16=0  Sync=0  Linked=0  [TranDis=0]  CmdQue=1
  [SPI: Clocking=0x0  QAS=0  IUS=0]
    length=66 (0x42)   Peripheral device type: disk
 Vendor identification: SCST_FIO
 Product identification: disk01
 Product revision level:  300
 Unit serial number: 27cddc71

The next step is to verify the target group configuration. That is possible
by verifying whether the output of the sg_rtpg command matches the values
configured on the target:

# sg_rtpg /dev/sdb
Report target port groups:
  target port group id : 0x100 , Pref=1
    target port group asymmetric access state : 0x00
    T_SUP : 0, O_SUP : 0, LBD_SUP : 0, U_SUP : 1, S_SUP : 1, AN_SUP : 1, AO_SUP : 1
    status code : 0x02
    vendor unique status : 0x00
    target port count : 01
    Relative target port ids:
      0x01
  target port group id : 0x101 , Pref=0
    target port group asymmetric access state : 0x02
    T_SUP : 0, O_SUP : 0, LBD_SUP : 0, U_SUP : 1, S_SUP : 1, AN_SUP : 1, AO_SUP : 1
    status code : 0x02
    vendor unique status : 0x00
    target port count : 01
    Relative target port ids:
      0x02

The relative target port ID and the target port group ID for a certain path
can be queried e.g. as follows:

# sg_vpd -p di /dev/sdb
Device Identification VPD page:
  Addressed logical unit:
    designator type: T10 vendor identification,  code set: ASCII
      vendor id: SCST_FIO
      vendor specific: 27cddc71-disk01
    designator type: EUI-64 based,  code set: Binary
      0x3237636464633731
  Target port:
    designator type: Relative target port,  code set: Binary
      Relative target port: 0x1
    designator type: Target port group,  code set: Binary
      Target port group: 0x100


Initiator Support
.................

On Linux systems ALUA support is provided by the scsi_dh_alua kernel
driver in combination with the user space multipathd daemon. You will
have to modify at least the following in /etc/multipath.conf to enable
ALUA:

* hardware_handler      "1 alua"
* prio                  alua
* path_grouping_policy  group_by_prio
* path_checker          tur

Notes:
- Newer versions of multipathd support a parameter called
  "detect_prio". It can be more convenient to enable this parameter instead of
  setting the parameter "prio" to "alua" for only those LUNs that support ALUA.
- Older versions of multipathd (e.g. RHEL 5 and SLES 10 SP1) need
  'prio_callout "/sbin/mpath_prio_alua /dev/%n"' instead of 'prio alua'.

# multipath -ll
23237636464633731 dm-3 SCST_FIO,disk01
size=1.0G features='3 queue_if_no_path pg_init_retries 50' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=1 status=active
| `- 10:0:0:0   sdd 8:48  active ready running
`-+- policy='service-time 0' prio=130 status=enabled
  `- 11:0:0:0   sde 8:64  active ready running
23133326137346538 dm-4 SCST_FIO,disk02
size=1.0G features='3 queue_if_no_path pg_init_retries 50' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=130 status=active
| `- 10:0:0:2   sdn 8:208 active ready running
`-+- policy='service-time 0' prio=1 status=enabled
  `- 11:0:0:2   sdp 8:240 active ready running

The following information can be derived from the above output:
* That the hardware handler (hw_handler) has been set to "1 alua".
* That multipathd created two priority groups - one with priority 1 and one
  with priority 130.
* That the SRP path with SCSI host number 10 will be used for communication
  with LUN "disk01" and that the SRP path with SCSI host number 11 will be used
  for communication with LUN "disk02".

More information about how to configure the device mapper and the scsi_dh_alua
driver can be found in the manual of your Linux distribution ("man
multipath.conf", "man multipath" and "man multipathd").

Windows initiator systems support ALUA from Windows Server 2008 on. For more
information about ALUA support in Windows Server, see also:
* Microsoft, Windows Server 2008 R2 Multipath I/O Overview, MSDN
  (http://technet.microsoft.com/en-us/library/cc725907.aspx).
* Microsoft, Multipathing Support in Windows Server 2008, July 2008, MSDN
  (http://blogs.msdn.com/b/san/archive/2008/07/27/multipathing-support-in-windows-server-2008.aspx).
* Microsoft, ALUA MPIO Logo Test, MSDN
  (http://msdn.microsoft.com/en-us/library/gg607458%28v=vs.85%29.aspx).


VAAI
----

SCST supports all 3 VAAI SCSI commands: WRITE SAME, COMPARE AND WRITE
(ATS) and EXTENDED COPY. Additionally, it supports not directly related
to VAAI Thin Provisioning capabilities, particularly, UNMAP SCSI
commands, WRITE SAME with UNMAP bit as well as thin provisioning related
devices' sysfs attributes (see above).

In some cases dev handlers should perform some manual actions to fully
benefit from SCST VAAI implementation. Those actions described in the
implementation notes below. For vdisk and fileio_tgt handlers they have
already been implemented.

IMPORTANT: To use EXTENDED COPY command between LUNs (datastores) they all
=========  MUST have the same PRODUCT IDENTIFICATION INQUIRY field. By
           default, to simplify remote devices identification, SCST uses
           vdisk names as PRODUCT IDENTIFICATION, so SCST devices look
	   differently from the initiators. However, for some reasons,
	   VMware does not use EXTENDED COPY between LUNs with different
	   PRODUCT IDENTIFICATION. Thus, to be able to use full VAAI in
	   your VMware setups you must manually set PRODUCT
	   IDENTIFICATION for all your VMware LUNs to the same value,
	   for instance, "SCST", via using "prod_id" attribute. It could
	   be done either by adding "prod_id" attribute to scstadmin
	   scst.conf, or by directly writing to SCST sysfs attribute.
	   For example:

	   HANDLER vdisk_blockio {
		DEVICE blockio1 {
		filename /dev/sda5
		prod_id SCST
	   }

	   or

	   echo SCST >/sys/kernel/scst_tgt/devices/blockio1/prod_id
	   correspondingly.

	   Note, this prod_id modification must be done on all
	   datastores BEFORE VMware connects to them.


Implementation notes
....................

WRITE SAME
~~~~~~~~~~

WRITE SAME command supports 2 modes:

1. Manual writing mode. In this mode WRITE SAME generates a set of
internal WRITE(16) SCSI commands to perform requested writing.

2. Remap mode. In this mode a dev handler, if supported, can remap being
written blocks to a single block and then tell SCST to manually write
parts of the requested area, which for some reason can not be remapped.

In both cases dev handlers should call from WRITE SAME command handler
scst_write_same() function. This function as the second argument gets
array of descriptors where to write the requested block of data. Last
element in this array must have len 0. If this argument is NULL, then
the whole area will be manually written by SCST. This value should be
used by dev handlers not supporting remapping blocks.

User space dev handlers should use SCST_EXEC_REPLY_DO_WRITE_SAME
reply_type of SCST_USER_EXEC subcommand. See scst_user doc for more
info.


COMPARE AND WRITE
~~~~~~~~~~~~~~~~~

COMPARE AND WRITE implemented by SCST a set of read, compare and write
actions done in atomic manner against affected blocks as well as regular
RESERVE SCSI commands. Particularly, COMPARE AND WRITE doesn't need any
queue flushing and unlimited number of COMPARE AND WRITE commands on
different blocks can be executed simultaneously.

The read and write actions implemented as generation of internal
READ(16) and WRITE(16) SCSI commands.

COMPARE AND WRITE command is completely transparent to dev handlers
(they only see the corresponding READ(16) and WRITE(16) commands), so
doesn't require any manual actions from them.


EXTENDED COPY
~~~~~~~~~~~~~

SCST implements EXTENDED COPY via internal Copy Manager target. This
target has the following specific attribute in its sysfs:

 - allow_not_connected_copy - if not set (default), an initiator can
perform copy only between devices it has direct access to via any
target/session. If set, any initiator can copy between any devices in
the system.

The Copy Manager has access only to those devices, for which it has LUNs
in /sys/kernel/scst_tgt/targets/copy_manager/copy_manager_tgt/luns/.
Devices from scst_vdisk dev handler added to it automatically upon
registration, but for other devices you need to manually add LUNs there
the same way as for any target driver. You can also delete any device at
any time from the Copy Manager visibility by deleting the corresponding
LUN from the sysfs. It might be useful during ALUA state switching.

Internally SCST implements EXTENDED COPY as generation of sets of
internal READ(16) and WRITE(16) SCSI commands. Dev handlers don't need
any manual actions to use it.

Also SCST provides for dev handlers possibility to remap blocks instead
of copy them, if they support this feature. It allows them to perform
EXTENDED COPY command much faster by just metadata update of their
backend storage, which supposed to be nearly instantaneous.

To use this feature, a dev handler should setup ext_copy_remap()
callback in its struct scst_dev_type. This callback is called by SCST
during EXTENDED COPY command processing to let the dev handler try to
remap affected blocks at first.

Upon finish, the dev handler should call scst_ext_copy_remap_done(). In
case of error, the dev handler should set the corresponding sense to cmd
and then also call scst_ext_copy_remap_done(cmd, NULL, 0).

If dev handler is not able to remap any part of the segment, if should
kmalloc(), then fill all leftover subsegments and supply them to
scst_ext_copy_remap_done(). SCST then will copy the subsegments using
internal copy machine, then kfree() the supplied array. If the dev
handler is not able to remap the whole segment, it can simply directly
supply the original segment to scst_ext_copy_remap_done().

It is highly recommended that in normal circumstances dev handlers call
scst_ext_copy_remap_done() from another thread context than one where
ext_copy_remap() callback was originally called, because otherwise there
could be recursion in the segments processing. Hopefully, this thread
context switch is natural for such potentially long operation as
EXTENDED COPY.


VMware and Ceph RBD space reclaim
---------------------------------

VMware with VMFS5 filesystem ignores UNMAP alignment, so if you use 4MB
Ceph RBD objects and VMFS5, only some discards will reclaim RBD space
due to 1MB discard not often hitting the tail of objects.

Thus, to have efficient ESXi space reclamation with RBD and VMFS5, you are
recommended to use 1 MB object size in Ceph.

See https://sourceforge.net/p/scst/mailman/message/35287598 thread for
details.


Caching
-------

By default for performance reasons VDISK FILEIO devices use write back
caching policy.

Generally, write back caching is safe for use and danger of it is
greatly overestimated, because most modern (especially, Enterprise
level) applications are well prepared to work with write back cached
storage. Particularly, such are all transactions-based applications.
Those applications flush cache to completely avoid ANY data loss on a
crash or power failure. For instance, journaled file systems flush cache
on each meta data update, so they survive power/hardware/software
failures pretty well.

Since locally on initiators write back caching is always on, if an
application cares about its data consistency, it does flush the cache
when necessary or on any write, if open files with O_SYNC. If it doesn't
care, it doesn't flush the cache. As soon as the cache flushes
propagated to the storage, write back caching on it doesn't make any
difference. If application doesn't flush the cache, it's doomed to loose
data in case of a crash or power failure doesn't matter where this cache
located, locally or on the storage.

To illustrate that consider, for example, a user who wants to copy /src
directory to /dst directory reliably, i.e. after the copy finished no
power failure or software/hardware crash could lead to a loss of the
data in /dst. There are 2 ways to achieve this. Let's suppose for
simplicity cp opens files for writing with O_SYNC flag, hence bypassing
the local cache.

1. Slow. Make the device behind /dst working in write through caching
mode and then run "cp -a /src /dst".

2. Fast. Let the device behind /dst working in write back caching mode
and then run "cp -a /src /dst; sync". The reliability of the result is
the same, but it's much faster than (1). Nobody would care if a crash
happens during the copy, because after recovery simply leftovers from
the not completed attempt would be deleted and the operation would be
restarted from the very beginning.

So, you can see in (2) there is no danger of ANY data loss from the
write back caching. Moreover, since on practice cp doesn't open files
for writing with O_SYNC flag, to get the copy done reliably, sync
command must be called after cp anyway, so enabling write back caching
wouldn't make any difference for reliability.

Also you can consider it from another side. Modern HDDs have at least
16MB of cache working in write back mode by default, so for a 10 drives
RAID it is 160MB of a write back cache. How many people are happy with
it and how many disabled write back cache of their HDDs? Almost all and
almost nobody correspondingly? Moreover, many HDDs lie about state of
their cache and report write through while working in write back mode.
They are also successfully used.

Note, Linux I/O subsystem guarantees to propagated cache flushes to the
storage only using data protection barriers, which usually turned off by
default (see http://lwn.net/Articles/283161). Without barriers enabled
Linux doesn't provide a guarantee that after sync()/fsync() all written
data really hit permanent storage. They can be stored in the cache of
your backstorage devices and, hence, lost on a power failure event.
Thus, ever with write-through cache mode, you still either need to
enable barriers on your backend file system on the target (for direct
/dev/sdX devices this is, indeed, impossible), or need a good UPS to
protect yourself from not committed data loss. Some info about barriers
from the XFS point of view could be found at
http://xfs.org/index.php/XFS_FAQ#Write_barrier_support. On Linux
initiators for Ext3 and ReiserFS file systems the barrier protection
could be turned on using "barrier=1" and "barrier=flush" mount options
correspondingly. You can check if the barriers turn on or off by looking
in /proc/mounts. Windows and, AFAIK, other UNIX'es don't need any
special explicit options and do necessary barrier actions on write-back
caching devices by default.

To limit this data loss with write back caching you can use files in
/proc/sys/vm to limit amount of unflushed data in the system cache.

If you for some reason have to use VDISK FILEIO devices in write through
caching mode, don't forget to disable internal caching on their backend
devices or make sure they have additional battery or supercapacitors
power supply on board. Otherwise, you still on a power failure would
loose all the unsaved yet data in the devices internal cache.

Note, on some real-life workloads write through caching might perform
better, than write back one with the barrier protection turned on.


Errors caching
..............

When using virtual device in FILEIO mode, the Linux page cache comes
into picture. The negative side of it is that it's sometimes also
caching errored pages. That is, if the underlying file experiences IO
errors, those errors might be cached by the Linux page cache. As a
result, even when the underlying file recovers and stops failing IOs,
the initiator may still hit IO errors returned by the Linux page cache,
until the cache re-reads the errored pages (usually it happens pretty
soon, but not immediately). To make sure that cached pages are dropped,
one of the following can be done:

- Detach the SCSI virtual device (del_device) and re-attach it
  (add_device). This should evict all the cached pages, unless somebody
  else holds the same "filename" opened.

- Issue a BLKFLSBUF ioctl to the same "filename" you provided for "add_device".

For the second option, a rudimentary C code is required:

fd = open(filename, O_RDWR);
if (fd < 0) {
    err = errno;
    ...
} else {
   err = ioctl(fd, BLKFLSBUF);
   if (err < 0) {
       err = errno;
       ...
   }
   close(fd);
}


BLOCKIO VDISK mode
------------------

This module works best for these types of scenarios:

1) Data that are not aligned to 4K sector boundaries and <4K block sizes
are used, which is normally found in virtualization environments where
operating systems start partitions on odd sectors (Windows and it's
sector 63).

2) Large block data transfers normally found in database loads/dumps and
streaming media.

3) Advanced relational database systems that perform their own caching
which prefer or demand direct IO access and, because of the nature of
their data access, can actually see worse performance with
non-discriminate caching.

4) Multiple layers of targets were the secondary and above layers need
to have a consistent view of the primary targets in order to preserve
data integrity which a page cache backed IO type might not provide
reliably.

Also it has an advantage over FILEIO that it doesn't copy data between
the system cache and the commands data buffers, so it saves a
considerable amount of CPU power and memory bandwidth.

IMPORTANT: Since data in BLOCKIO and FILEIO modes are not consistent between
=========  each other, if you try to use a device in both those modes
	   simultaneously, you will almost instantly corrupt your data
	   on that device.

IMPORTANT: If SCST 1.x BLOCKIO worked by default in NV_CACHE mode, when
=========  each device reported to remote initiators as having write through
           caching. But if your backend block device has internal write
	   back caching it might create a possibility for data loss of
	   the cached in the internal cache data in case of a power
	   failure. Starting from SCST 2.0 BLOCKIO works by default in
	   non-NV_CACHE mode, when each device reported to remote
	   initiators as having write back caching, and synchronizes the
	   internal device's cache on each SYNCHRONIZE_CACHE command
	   from the initiators. It might lead to some *PERFORMANCE LOSS*,
	   so if you are are sure in your power supply and want to
	   restore the 1.x behavior, your should recreate your BLOCKIO
	   devices in NV_CACHE mode.


Pass-through mode
-----------------

In the pass-through mode (i.e. using the pass-through device handlers
scst_disk, scst_tape, etc) SCSI commands, coming from remote initiators,
are passed to local SCSI devices on target as is, without any
modifications.

SCST supports 1 to many pass-through, when several initiators can safely
connect a single pass-through device (a tape, for instance). For such
cases SCST emulates all the necessary functionality.

In the sysfs interface all real SCSI devices are listed in
/sys/kernel/scst_tgt/devices in form host:channel:id:lun numbers, for
instance 1:0:0:0. The recommended way to match those numbers to your
devices is use of lsscsi utility.

Each pass-through dev handler has in its root subdirectory
/sys/kernel/scst_tgt/handlers/handler_name, e.g.
/sys/kernel/scst_tgt/handlers/dev_disk, "mgmt" file. It allows the
following commands. They can be sent to it using, e.g., echo command.

 - "add_device" - this command assigns SCSI device with
host:channel:id:lun numbers to this dev handler.

echo "add_device 1:0:0:0" >/sys/kernel/scst_tgt/handlers/dev_disk/mgmt

will assign SCSI device 1:0:0:0 to this dev handler.

 - "del_device" - this command unassigns SCSI device with
host:channel:id:lun numbers from this dev handler.

As usually, on read the "mgmt" file returns small help about available
commands.

You need to manually assign each your real SCSI device to the
corresponding pass-through dev handler using the "add_device" command,
otherwise the real SCSI devices will not be visible remotely. The
assignment isn't done automatically, because it could lead to the
pass-through dev handlers load and initialization problems if any of the
local real SCSI devices are malfunctioning.

As any other hardware, the local SCSI hardware can not handle commands
with amount of data and/or segments count in scatter-gather array bigger
some values. Therefore, when using the pass-through mode you should note
that values for maximum number of segments and maximum amount of
transferred data (max_sectors) for each SCSI command on devices on
initiators can not be bigger, than corresponding values of the
corresponding SCSI devices on the target. Otherwise you will see
symptoms like small transfers work well, but large ones stall and
messages like: "Unable to complete command due to SG IO count
limitation" are printed in the kernel logs.

You can't control from the user space limit of the scatter-gather
segments, but for block devices usually it is sufficient if you set on
the initiators /sys/block/DEVICE_NAME/queue/max_sectors_kb in the same
or lower value as in /sys/block/DEVICE_NAME/queue/max_hw_sectors_kb for
the corresponding devices on the target.

For not-block devices SCSI commands are usually generated directly by
applications, so, if you experience large transfers stalls, you should
check documentation for your application how to limit the transfer
sizes.

Another way to solve this issue is to build SG entries with more than 1
page each. See the following patch as an example:
http://scst.sourceforge.net/sgv_big_order_alloc.diff


Performance
-----------

SCST from the very beginning has been designed and implemented to
provide the best possible performance. Since there is no "one fit all"
the best performance configuration for different setups and loads, SCST
provides extensive set of settings to allow to tune it for the best
performance in each particular case. You don't have to necessary use
those settings. If you don't, SCST will do very good job to autotune for
you, so the resulting performance will, in average, be better
(sometimes, much better) than with other SCSI targets. But in some cases
you can by manual tuning improve it even more.

Before doing any performance measurements note that performance results
are very much dependent from your type of load, so it is crucial that
you choose access mode (FILEIO, BLOCKIO, O_DIRECT, pass-through), which
suits your needs the best.

In order to get the maximum performance you should:

1. For SCST:

 - Disable CONFIG_SCST_STRICT_SERIALIZING, CONFIG_SCST_EXTRACHECKS,
   CONFIG_SCST_TRACING, CONFIG_SCST_DEBUG*, CONFIG_SCST_STRICT_SECURITY,
   CONFIG_SCST_MEASURE_LATENCY

2. For target drivers:

 - Disable in Makefiles CONFIG_SCST_EXTRACHECKS, CONFIG_SCST_TRACING,
   CONFIG_SCST_DEBUG*

3. For device handlers, including VDISK:

 - Disable in Makefile CONFIG_SCST_TRACING and CONFIG_SCST_DEBUG.

Note, by disabling CONFIG_SCST_TRACING and CONFIG_SCST_DEBUG you are
disabling many useful SCST diagnostic messages, which can significantly
help in many troubleshooting cases. So, if you may consider to keep
CONFIG_SCST_TRACING, its performance impact is very limited.

4. Make sure you have io_grouping_type option set correctly, especially
in the following cases:

 - Several initiators share your target's backstorage. It can be a
   shared LU using some cluster FS, like VMFS, as well as can be
   different LUs located on the same backstorage (RAID array). For
   instance, if you have 3 initiators and each of them using its own
   dedicated FILEIO device file from the same RAID-6 array on the
   target.

   In this case for the best performance you should have
   io_grouping_type option set in value "never" in all the LUNs' targets
   and security groups.

 - Your initiator connected to your target in MPIO mode. In this case for
   the best performance you should:

    * Either connect all the sessions from the initiator to a single
      target or security group and have io_grouping_type option set in
      value "this_group_only" in the target or security group,

    * Or, if it isn't possible to connect all the sessions from the
      initiator to a single target or security group, assign the same
      numeric io_grouping_type value for each target/security group this
      initiator connected to. The exact value itself doesn't matter,
      important only that all the targets/security groups use the same
      value.

Don't forget, io_grouping_type makes sense only if you use CFQ I/O
scheduler on the target and for devices with threads_num >= 0 and, if
threads_num > 0, with threads_pool_type "per_initiator".

You can check if in your setup io_grouping_type set correctly as well as
if the "auto" io_grouping_type value works for you by tests like the
following:

 - For not MPIO case you can run single thread sequential reading, e.g.
   using buffered dd, from one initiator, then run the same single
   thread sequential reading from the second initiator in parallel. If
   io_grouping_type is set correctly the aggregate throughput measured
   on the target should only slightly decrease as well as all initiators
   should have nearly equal share of it. If io_grouping_type is not set
   correctly, the aggregate throughput and/or throughput on any
   initiator will decrease significantly, in 2 times or even more. For
   instance, you have 80MB/s single thread sequential reading from the
   target on any initiator. When then both initiators are reading in
   parallel you should see on the target aggregate throughput something
   like 70-75MB/s with correct io_grouping_type and something like
   35-40MB/s or 8-10MB/s on any initiator with incorrect.

 - For the MPIO case it's quite easier. With incorrect io_grouping_type
   you simply won't see performance increase from adding the second
   session (assuming your hardware is capable to transfer data through
   both sessions in parallel), or can even see a performance decrease.

5. If you are going to use your target in an VM environment, for
instance as a shared storage with VMware, make sure all your VMs
connected to the target via *separate* sessions. For instance, for iSCSI
it means that each VM has own connection to the target, not all VMs
connected using a single connection. You can check it using SCST sysfs
interface. For other transports you should use available facilities,
like NPIV for Fibre Channel, to make separate sessions for each VM. If
you miss it, you can greatly loose performance of parallel access to
your target from different VMs. This isn't related to the case if your
VMs are using the same shared storage, like with VMFS, for instance. In
this case all your VM hosts will be connected to the target via separate
sessions, which is enough.

6. For other target and initiator software parts:

 - Make sure you applied on your kernel all available SCST patches.
   If for your kernel version this patch doesn't exist, it is strongly
   recommended to upgrade your kernel to version, for which this patch
   exists.

 - Don't enable debug/hacking features in the kernel, i.e. use them as
   they are by default.

 - The default kernel read-ahead and queuing settings are optimized
   for locally attached disks, therefore they are not optimal if they
   attached remotely (SCSI target case), which sometimes could lead to
   unexpectedly low throughput. You should increase read-ahead size to at
   least 512KB or even more on all initiators and the target.

   You should also limit on all initiators maximum amount of sectors per
   SCSI command. This tuning is also recommended on targets with large
   read-ahead values. To do it on Linux, run:

   echo “64” > /sys/block/sdX/queue/max_sectors_kb

   where specify instead of X your imported from target device letter,
   like 'b', i.e. sdb.

   To increase read-ahead size on Linux, run:

   blockdev --setra N /dev/sdX

   where N is a read-ahead number in 512-byte sectors and X is a device
   letter like above.

   Note: you need to set read-ahead setting for device sdX again after
   you changed the maximum amount of sectors per SCSI command for that
   device.

   Note2: you need to restart SCST after you changed read-ahead settings
   on the target. It is a limitation of the Linux read ahead
   implementation. It reads RA values for each file only when the file
   is open and not updates them when the global RA parameters changed.
   Hence, the need for vdisk to reopen all its files/devices.

 - You may need to increase amount of requests that OS on initiator
   sends to the target device. To do it on Linux initiators, run

   echo “64” > /sys/block/sdX/queue/nr_requests

   where X is a device letter like above.

   You may also experiment with other parameters in /sys/block/sdX
   directory, they also affect performance. If you find the best values,
   please share them with us.

 - On the target use CFQ IO scheduler. In most cases it has performance
   advantage over other IO schedulers, sometimes huge (2+ times
   aggregate throughput increase).

 - It is recommended to turn the kernel preemption off, i.e. set
   the kernel preemption model to "No Forced Preemption (Server)".

 - Looks like XFS is the best filesystem on the target to store device
   files, because it allows considerably better linear write throughput,
   than ext3.

7. For hardware on target.

 - Make sure that your target hardware (e.g. target FC or network card)
   and underlying IO hardware (e.g. IO card, like SATA, SCSI or RAID to
   which your disks connected) don't share the same PCI bus. You can
   check it using lspci utility. They have to work in parallel, so it
   will be better if they don't compete for the bus. The problem is not
   only in the bandwidth, which they have to share, but also in the
   interaction between cards during that competition. This is very
   important, because in some cases if target and backend storage
   controllers share the same PCI bus, it could lead up to 5-10 times
   less performance, than expected. Moreover, some motherboard (by
   Supermicro, particularly) have serious stability issues if there are
   several high speed devices on the same bus working in parallel. If
   you have no choice, but PCI bus sharing, set in the BIOS PCI latency
   as low as possible.

8. If you use VDISK IO module in FILEIO mode, NV_CACHE option will
provide you the best performance. But using it make sure you use a good
UPS with ability to shutdown the target on the power failure.

Baseline performance numbers you can find in those measurements:
http://lkml.org/lkml/2009/3/30/283.

IMPORTANT: If you use on initiator some versions of Windows (at least W2K)
=========  you can't get good write performance for VDISK FILEIO devices with
           default 512 bytes block sizes. You could get about 10% of the
	   expected one. This is because of the partition alignment, which
	   is (simplifying) incompatible with how Linux page cache
	   works, so for each write the corresponding block must be read
	   first. Use 4096 bytes block sizes for VDISK devices and you
	   will have the expected write performance. Actually, any OS on
	   initiators, not only Windows, will benefit from block size
	   max(PAGE_SIZE, BLOCK_SIZE_ON_UNDERLYING_FS), where PAGE_SIZE
	   is the page size, BLOCK_SIZE_ON_UNDERLYING_FS is block size
	   on the underlying FS, on which the device file located, or 0,
	   if a device node is used. Both values are from the target.
	   See also important notes about setting block sizes >512 bytes
	   for VDISK FILEIO devices above.


9. In some cases, for instance working with SSD devices, which consume
100% of a single CPU load for data transfers in their internal threads,
to maximize IOPS it can be needed to assign for those threads dedicated
CPUs. Consider using cpu_mask attribute for devices with
threads_pool_type "per_initiator" or Linux CPU affinity facilities for
other threads_pool_types. No IRQ processing should be done on those
CPUs. Check that using /proc/interrupts. See taskset command and
Documentation/IRQ-affinity.txt in your kernel's source tree for how to
assign IRQ affinity to tasks and IRQs.

The reason for that is that processing of coming commands in SIRQ
context might be done on the same CPUs as SSD devices' threads doing data
transfers. As the result, those threads won't receive all the processing
power of those CPUs and perform worse.

10. If your storage is capable of operation on hundreds of thousands
IOPS level, you can use poll_us sysfs attribute to set how many us each
SCST thread is polling its queue after it became empty in a hope that a
new command can come. In some cases, polling can significantly increase
IOPS, especially if low power states on CPU not disabled, because on
high IOPS polling could be cheaper comparing to spending significant
time on entering, then exiting CPU low power states + corresponding
context switches. Polling is disabled by default. The recommended value
to start from is 5-10 us. Then you can increase or decrease it to see if
your IOPS are increasing or decreasing.


Commands suspending takes too long
----------------------------------

SCST is suspending commands during some management activities like
adding/deleting LUNs or devices. It is done to have lockless LUNs
translation on the hot commands processing path. This brings significant
performance advantage. You will see a message like "Waiting for X active
commands to complete" when this wait started.

But downside of it is that no new commands start executing until older
ones, which had started before the suspending begun, finished. This
wait can not be any longer, than the worst command latency any your
initiator is seeing at this particular time.

So, if this wait takes too long, in majority of cases it means that you
are overloading your storage. A proper storage should have worst case
latency below few hundreds of milliseconds. In this case the SCST
suspending will finish in few hundreds of milliseconds at worse.

Another case, when it can take too long to suspend is a hung user space
device (i.e. scst_user device) not responding to any command. In this
case you should kill the corresponding user space program to finish
suspending.


Work if target's backstorage or link is too slow
------------------------------------------------

Under high I/O load, when your target's backstorage gets overloaded, or
working over a slow link between initiator and target, when the link
can't serve all the queued commands on time, you can experience I/O
stalls or see in the kernel log abort or reset messages.

At first, consider the case of too slow target's backstorage. On some
seek intensive workloads even fast disks or RAIDs, which able to serve
continuous data stream on 500+ MB/s speed, can be as slow as 0.3 MB/s.
Another possible cause for that can be MD/LVM/RAID on your target as in
http://lkml.org/lkml/2008/2/27/96 (check the whole thread as well).

Thus, in such situations simply processing of one or more commands takes
too long time, hence initiator decides that they are stuck on the target
and tries to recover. Particularly, it is known that the default amount
of simultaneously queued commands (48) is sometimes too high if you do
intensive writes from VMware on a target disk, which uses LVM in the
snapshot mode. In this case value like 16 or even 8-10 depending of your
backstorage speed could be more appropriate.

There are 6 possible actions, which you can do to workaround or fix such
issues:

1. Ignore incoming task management (TM) commands. It's fine if there are
not too many of them, so average performance isn't hurt and the
corresponding device isn't getting put offline, i.e. if the backstorage
isn't a way too slow.

2. Decrease /sys/block/sdX/device/queue_depth on the initiator in case
if it's Linux (see below how) or/and SCST_MAX_TGT_DEV_COMMANDS constant
in scst_priv.h file until you stop seeing incoming TM commands.
ISCSI-SCST driver also has its own iSCSI specific parameter for that,
see its README file.

To decrease device queue depth on Linux initiators you can run command:

# echo Y >/sys/block/sdX/device/queue_depth

where Y is the new number of simultaneously queued commands, X - your
imported device letter, like 'a' for sda device. There are no special
limitations for Y value, it can be any value from 1 to possible maximum
(usually, 32), so start from dividing the current value on 2, i.e. set
16, if /sys/block/sdX/device/queue_depth contains 32.

3. Increase the corresponding timeout on the initiator. For Linux it is
located in
/sys/devices/platform/host*/session*/target*:0:0/*:0:0:1/timeout. It can
be done automatically by an udev rule. For instance, the following
rule will increase it to 300 seconds:

SUBSYSTEM=="scsi", KERNEL=="[0-9]*:[0-9]*", ACTION=="add", ATTR{type}=="0|7|14", ATTR{timeout}="300"

By default, this timeout is 30 or 60 seconds, depending on your distribution.

4. Try to avoid such seek intensive workloads.

5. Increase speed of the target's backstorage.

6. Implement in SCST QoS, so queue depth size on the target is
dynamically adjusted, hence worst case initiator seen latencies are
controlled.

Next, consider the case of too slow link between initiator and target,
when the initiator tries to simultaneously push N commands to the target
over it. In this case time to serve those commands, i.e. send or receive
data for them over the link, can be more, than timeout for any single
command, hence one or more commands in the tail of the queue can not be
served on time less than the timeout, so the initiator will decide that
they are stuck on the target and will try to recover.

To workaround/fix this issue in this case you can use ways 1, 2, 3 above
or (7): increase speed of the link between target and initiator.

Note, that logged messages about QUEUE_FULL status are quite different
by nature. This is a normal work, just SCSI flow control in action.
Simply don't enable "mgmt_minor" logging level, or, alternatively, if
you are confident in the worst case performance of your back-end storage
or initiator-target link, you can increase SCST_MAX_TGT_DEV_COMMANDS in
scst_priv.h to 64. Usually initiators don't try to push more commands on
the target.


Credits
-------

Thanks to:

 * Mark Buechler <mark.buechler@gmail.com> for a lot of useful
   suggestions, bug reports and help in debugging.

 * Ming Zhang <mingz@ele.uri.edu> for fixes and comments.

 * Nathaniel Clark <nate@misrule.us> for fixes and comments.

 * Calvin Morrow <calvin.morrow@comcast.net> for testing and useful
   suggestions.

 * Hu Gang <hugang@soulinfo.com> for the original version of the
   LSI target driver.

 * Erik Habbinga <erikhabbinga@inphase-tech.com> for fixes and support
   of the LSI target driver.

 * Ross S. W. Walker <rswwalker@hotmail.com> for BLOCKIO inspiration
   and Vu Pham <huongvp@yahoo.com> who implemented it for VDISK dev handler.

 * Alessandro Premoli <a.premoli@andxor.it> for fixes

 * Nathan Bullock <nbullock@yottayotta.com> for fixes.

 * Terry Greeniaus <tgreeniaus@yottayotta.com> for fixes.

 * Krzysztof Blaszkowski <kb@sysmikro.com.pl> for many fixes and bug reports.

 * Jianxi Chen <pacers@users.sourceforge.net> for fixing problem with
   devices >2TB in size

 * Bart Van Assche <bvanassche@acm.org> for a lot of help

 * Daniel Debonzi <debonzi@linux.vnet.ibm.com> for a big part of the
   initial SCST sysfs tree implementation


Vladislav Bolkhovitin <vst@vlnb.net>, http://scst.sourceforge.net
