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## Generic SCSI Target Subsystem for Linux

# Contributing to SCST

If you would like to contribute to SCST development, you can do in many ways:

- By sending donations. They will be spent on further work making SCST better, including buying new hardware, as well as on providing better support and troubleshooting for you. If you want to donate another amount, than listed on the provided buttons, you can directly edit URL they are pointing to.
- By sending patches, which fix bugs or implement new functionality. See below a list of possible SCST improvements with some possible implementation ideas.
- By writing or updating various documentation to keep it complete and up to date. For instance, SCST internals description document is in some areas quite outdated. Particularly, many functions were renamed since time, when it was written. It would be good to bring it up to date.
- By reporting bugs or other problems.

# Possible SCST extensions and improvements

### Asynchronous FILEIO in scst_vdisk handler

At the moment scst_vdisk handler for FILEIO uses regular synchronous read/write() calls and makes deep queue depth by using multiple threads. This is not too high performance model of operations. It would be much better to use asynchronous I/O with not blocking I/O calls.

In the user space native AIO is available for many years, but only very recently ability to use it was added in the kernel. Changing FILEIO to use the new interface should significantly (up to multiple times) increase performance of FILEIO devices.

### Support for O_DIRECT in scst_vdisk handler

At the moment, scst_vdisk handler doesn't support O_DIRECT option and possibility to set it was disabled. This limitation caused by Linux kernel expectation that memory supplied to read() and write() functions with O_DIRECT flag is mapped to some user space application. Having O_DIRECT together with above asynchronous FILEIO would be another significant performance boost for modern solid state devices. For instance, in fio utility direct AIO long ago proven to be the fastest way to benchmark storage.

It is relatively easy to remove that limitation. Function dio_refill_pages() should be modified to check before calling get_user_pages() if current->mm is not NULL. If it is NULL, then, instead of calling get_user_pages(), dio->pages should be filled by pages, taken directly from dio->curr_user_address. Each such page should be referenced by page_cache_get(). That's all.

### Solve SG IO count limitation issue in pass-through mode

In the pass-through mode (i.e. using the pass-through device handlers like scst_tape, etc.) SCSI commands, coming from remote initiators, are passed to local SCSI hardware on target as is, without any modifications. 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. For some commands SCST can split them on subcommands and, hence, workaround this problem, but it isn't always possible. For instance, for tapes splitting write commands may mean corrupting the tape data.

If you have this issue you will see symptoms like small transfers work well, but large transfers stall and messages like: "Unable to complete command due to SG IO count limitation" are printed in the kernel logs.

The only complete way to fix this problem is to allocate data buffers with number of entries inside the SG IO count limitation. In sgv_big_order_alloc.diff you can find a possible way to solve this issue.

You can also look at patch sgv_big_order_alloc-sfw5-rc3.diff created by Frank Zago for SCST 2.0.0. It was submitted too late to be included in it. Update for SCST trunk is welcome!

Note, scst_disk handler already implements a workaround for it.

### Memory registration

In some cases a target driver might need to register memory used for data buffers in the hardware. At the moment, none of SCST target drivers, including InfiniBand SRP target driver, need that feature. But in case if in future there is a need in such a feature, it can be easily added by extending SCST SGV cache. The SCST SGV cache is a memory management subsystem in SCST. It doesn't free to the system each data buffer, which is not used anymore, but keeps it for a while to let it be reused by the next consecutive command to reduce command processing latency and, hence, improve performance.

To support memory buffers registrations, it can be extended by the following way:

1. Struct scst_tgt_template would be extended to have 2 new callbacks:

- int register_buffer(struct scst_cmd *cmd)
- int unregister_buffer(unsigned long mem_priv, void *scst_priv)

2. SCST core would be extended to have 4 new functions:

- int scst_mem_registered(struct scst_cmd *cmd)
- int scst_mem_deregistered(void *scst_priv)
- int scst_set_mem_priv(struct scst_cmd *cmd, unsigned long mem_priv)
- unsigned long scst_get_mem_priv(struct scst_cmd *cmd)

3. The workflow would be the following:

1. If target driver defined register_buffer() and unregister_buffer() callbacks, SCST core would allocate a dedicated SGV cache for each instance of struct scst_tgt, i.e. target.
2. When there would be an SGV cache miss in memory buffer for a command allocation, SCST would check if register_buffer() callback was defined in the target driver's template and, if yes, would call it.
3. In register_buffer() callback the target driver would do necessary actions to start registration of the commands memory buffer.
4. Upon register_buffer() callback returns, SCST core would suspend processing the corresponding command and would switch to the next commands processing.
5. After the memory registration finished, the target driver would call scst_set_mem_priv() to associate the memory buffer with some internal data.
6. Then the target driver would call scst_mem_registered() and SCST would resume processing the command. Functions scst_set_mem_priv() and scst_mem_registered() can be called from inside register_buffer(). In this case SCST core would continue processing the command immediately without suspending.
7. After the command finished, the corresponding memory buffer would remain in the SGV cache in the registered state and would be reused by the next commands. For each of them the target driver can at any time figure out the associated with the registered buffer data by using scst_get_mem_priv().
8. When the SGV cache decide that there is a time to free the memory buffer, it would call the target driver's unregister_buffer() callback.
9. In this callback the target driver would do necessary actions to start deregistration of the commands memory buffer.
10. Upon unregister_buffer() callback returns, SGV cache would suspend freeing the corresponding buffer and would switch to other deals it has.
11. After the memory deregistration finished, the target driver would call scst_mem_deregistered() and pass to it scst_priv pointer, received in unregister_buffer(). Then the memory buffer would be freed by the SGV cache. Function scst_mem_deregistered() can be called from inside unregister_buffer(). In this case SGV cache would free the buffer immediately without suspending.

### SCST usage with non-SCSI transports

SCST might also be used with non-SCSI speaking transports, like NBD or AoE. Such cooperation would allow them to use SCST-emulated backend.

For user space targets this is trivial: they simply should use SCST-emulated devices locally via scst_local module.

For in-kernel non-SCSI target driver it's a bit more complicated. They should implement a small layer, which would translate their internal READ/WRITE requests to corresponding SCSI commands and, on the way back, SCSI status and sense codes to their internal status codes.

### GET CONFIGURATION command

SCSI command GET CONFIGURATION is mandatory for SCSI multimedia devices, like CD/DVD-ROMs or recorders, see MMC standard. Currently SCST lacks support for it, which leads to problems with some programs depending on the result of GET CONFIGURATION command execution.

It would be good to add support for it in the SCST core.