Contributing to SCST

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

  • By reporting bugs or other problems.
  • 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 For instance, SCST internals description document is in some areas quite outdated.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 sending patches, which fix bugs or implement new functionality. See below a list of possible SCST improvements with some possible implementation ideas.
  • By sending donations. They will be spent on making SCST even better as well as on providing better support and troubleshooting for you.

Possible SCST improvements

Zero-copy FILEIO for READ-direction commands

At the moment, SCST in FILEIO mode uses standard Linux read() and write() syscalls paths, which copy data from the page cache to the supplied buffer and back. Zero-copy FILEIO would use page cache data directly. This would be a major performance improvement, especially for fast hardware, like Infiniband, because it would eliminate the data copy latency as well as considerably ease CPU and memory bandwidth load. This proposal is limited for READs only, because for WRITEs it is a lot harder to implement, so it is worth to do zero-copy for READs and WRITEs separately.

The main idea is to add one more flag to filp_open() "flags" parameter (like O_RDONLY, O_DIRECT, etc.) O_ZEROCOPY, which would be available only if the caller is from the kernel space. In this case fd->f_op->readv(), do_sync_readv_writev(), etc. would receive as the pointer to data buffer not a real data buffer, but pointer to an empty SG vector. Then:

  • Generic buffer allocation in SCST would not be used, instead vdisk_parse() would allocate the SG vector, but wouldn't fill it with actual pages.
  • In generic_file_aio_read(), if O_ZEROCOPY flag was set, function do_generic_file_read() would be called with the last parameter set to a pointer to new function file_zero_copy_read_actor() instead of file_read_actor().
  • Function file_zero_copy_read_actor() would be basically the same as file_read_actor(), but, instead of copy data using __copy_to_user*() functions, it would add the supplied page to the appropriate place in the received in desc->arg.buf SG vector and reference, i.e. page_get(), that page.
  • In vdisk_devtype.on_free_cmd(), which doesn't exist yet, all pages from the SG vector would be dereferenced, i.e. page_put(). Then the SG vector itself would be freed.

That's all. For WRITEs the current code path would remain unchanged.

Zero-copy FILEIO for WRITE-direction commands

Implementation should be similar to zero-copy FILEIO for READ commands and should be done after it. All incoming data should be inserted in the page cache, then dereferenced in vdisk_devtype.on_free_cmd(). The main problem is insertion of data pages in the page cache, namely, locking issues related to it. They should be carefully investigated.

Persistent reservations

Support for PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT is required to work in many cluster environments, e.g. Windows 2003 Cluster.

For implementation you should use scst_reserve_local() and scst_release_local() as a base. You should store all reservation keys for in files in /var/scst, one file per device (it would allow to eliminate additional locking), like /var/scst/boot_disk for device "boot_disk" and load them in memory, when device would be registered.

In the first version it can be done for virtual devices only and reject PERSISTENT RESERVE IN and OUT commands for pass-through devices with "COMMAND NOT SUPPORTED" sense data.

Automatic sessions reassignment

At the moment, if security name for an initiator reassigned (moved) to another security group, the existing sessions from that initiator are not automatically reassigned to the new security group, i.e. they remain in the old one. The only ways to reassign them are either sessions restart, or restart of the corresponding target driver. Both in many cases are not options.

To implement that you should on event of any group change:

  • Globally suspend all activities by scst_suspend_activity().
  • Go over all existing sessions. For each find the corresponding ACG (see scst_init_session() as an example) and check if it's the same as the existing one. If it's the same, then go to the next session. Otherwise, reassign it to the new ACG. For that you should go over all devices in the group/session pair (tgt_dev's) and delete not existing in the new ACG tgt_dev's, add new ones and keep the existing ones.
  • Resume the activities.

Dynamic I/O flow control

At the moment, if an initiator or several initiators simultaneously send to target too many commands, especially in seek intensive workloads, target can get overloaded and not able to finish commands on time. In such cases you can see on the initiator(s) messages about aborting commands or resetting the target. See in SCST core README section "What if target's backstorage is too slow" for more details. To fix this problem it is necessary to implement a dynamic I/O flow control in SCST core.

The flow control, generally, is quite simple. Each SCST command has timeout value, which is set by the corresponding dev handler. SCST core should keep device's queue depth at the level that the worst command's execution time, i.e. time between scst_rx_cmd() and scst_finish_cmd(), would be between something like timeout/10 and timeout/5. So, commands execution time should be checked and:

  • If it's > timeout/5, then the new queue depth should be set to max(1, cur_depth/2)
  • If it's < timeout/10, then new queue depth should be set to min(MAX_DEPTH, cur_depth+1). This shouldn't be done too often, once in a few minutes should be sufficient

The above is, of course, an oversimplification to let you see the idea. Implementation considering real life cases should be as the following:

1. There are several parameters:

  • P - load watch period. During this period all the statistic is gathered and processed.
  • MN - underload ratio divisor, which sets the underload portion of timeout. If the longest execution time among all commands completed during period P is below timeout/MN, the corresponding device considered underloaded.
  • MX - overload ratio divisor, which sets the overload portion of timeout. If the longest execution time among all commands completed during period P is above timeout/MX, the corresponding device considered overloaded.
  • I - step on which device's queue size will be increased if device considered underloaded.
  • D - divisor on which device's queue size will be decreased if device considered overloaded.
  • QI - quick fall interval. See description of Q parameter.
  • Q - quick fall ratio divisor. If the longest execution time of a completed command is above timeout/Q and time from the previous quick fall is smaller than QI, the corresponding device considered heavily overloaded. The quick fall is needed to handle cases when load on device is instantly increased on the way, where it can't handle it properly.
  • QD - divisor on which device's queue size will be decreased if device considered heavily overloaded.

The default values should be something like: P=15 sec., MN=20, MX=10, Q=3, I=1, D=2, QI=5 sec., QD=10.

2. There are the following new variables in struct scst_device:

  • queue_depth - current queue depth.
  • max_exec_ratio - maximum commands timeout/(execution time).
  • queue_was_full - flag, marking that the queue was at least once full during period P.
  • quick_fall_time - time of the last quick fall.
  • flow_lock - protects flow control related variables, where needed.
  • ...

3. The commands processing path should be as the following:

  • In scst_rx_cmd() the start time of the command is recorded (already done).
  • In __scst_init_cmd(), if dev->dev_cmd_count == dev->queue_depth, dev->queue_was_full set to true.
  • In scst_finish_cmd() dev->max_exec_ratio set to max(dev->max_exec_ratio, (cmd's exec_time)*100/cmd->timeout).
  • If in scst_finish_cmd() cmd's exec time is above cmd->timeout/Q and time from the latest quick fall is above QI, then:
    • dev->queue_depth set to max(1, dev->queue_depth/QD).
    • Flow control period reset, i.e. started again, including setting dev->max_exec_ratio to 0 and dev->quick_fall_time to jiffies.

4. There should be a work, which once in a P seconds will check dev->max_exec_ratio, then:

  • If device neither underloaded, nor overloaded. i.e. max_exec_ratio between defined by MN and MX, do nothing.
  • If device was underloaded:
    • if dev->queue_was_full is false, then do nothing.
    • if dev->queue_was_full is true, then set dev->queue_depth to min(SCST_MAX_DEV_COMMANDS, dev->queue_depth + I).
  • If device was overloaded, then set dev->queue_depth to max(1, dev->queue_depth/D).

Then the flow control period is reset, i.e. started again, including setting dev->max_exec_ratio to 0 and dev->quick_fall_time to jiffies.

That's all. Then only support for initiators, like iSCSI, which don't handle QUEUE FULL to decrease amount of queued commands, should be added. Such initiators expect target to control size of the queue, via, e.g., through MAX_SN for iSCSI.

For it at the stage 2 of the dynamic flow control development the following should be done:

  • New callback on_queue_depth_adjustment() should be added to struct scst_tgt_template.
  • If target driver defined it, each time after dev->queue_depth changed on_queue_depth_adjustment() should be called. In this callback target driver should change internal queue_depth to, e.g. for iSCSI target, set max_sn in the replies correctly.

Then, at the latest stage of the development, logic to not schedule the flow control work on idle devices should be added.

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.

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.

Refactoring of command execution path in scst_vdisk handler

At the moment, in scst_vdisk handler command execution function vdisk_do_job() is overcomplicated and not very performance effective. It would be good to replace all those ugly "switch" statements by choosing the handler for each SCSI command by indirect function call on an array of function pointers.

I.e., there should be an array vdisk_exec_fns with 256 entries of function pointers:

void (*cmd_exec_fn) (struct scst_cmd *cmd)

Then vdisk_do_job() should look like

static int vdisk_do_job(struct scst_cmd *cmd) { return vdisk_exec_fns[cmd->cdb[0]](cmd); }

Solve SG IO count limitation issue in 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 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. If you have this issue 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.

In sgv_big_order_alloc.diff you can find a possible way to solve this issue.