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.
  • 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 would be spent on making SCST even better.

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. 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. 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