ISCSI-SCST is a forked (with all respects) version of IET with updates to work over SCST as well as with many improvements and bugfixes. The reason of fork is that the necessary changes are intrusive and with the current IET merge policy, where only simple bugfix-like patches, which doesn't touch the core code, could be merged, it is very unlikely that they will be merged in the main IET trunk.

ISCSI-SCST has the following major advantages over the mainline IET:

• It uses full SCST power without loosing any existing feature (except, maybe, "illegal" from SCSI specifications point of view MPIO). Namely, you can use with it:

• Pass-through mode with one to many relationship, i.e. when multiple initiators can connect to the exported pass-through devices. For instance, you can safely export your parallel SCSI tape or tape library on your iSCSI net and multiple initiators can share it without risk of data loss because of the shared usage. Existing "rawio" patch for IET supports only non-enforced 1:1 relationship, so it is unsafe to use it in multiple initiators environments.
  
• More advances devices visibility management, when different initiators can see different set of devices with different access permissions from the same target.
• O_DIRECT, i.e. "BLOCKIO on files", mode, which has all advantages of BLOCKIO, but also supports files on file systems. Sometimes, in the appropriate cases, this mode can make performance difference in 100% or even more.
• With 4KB blocks you can forget about abysmal write performance caused by misaligned partitions. All modern OS'es, including Windows starting from, at least, Windows 2000, work perfectly with 4KB block devices without any additional storage or handling overhead.
• Virtual CD/DVD-ROMs without necessity for manual patching.
• Ability to create target devices emulators in the user space.
• Ability to create multi-transport SCSI targets, which can export (possibly, the same) devices over multiple transports.

• It has many code improvements and cleanups, including stability and iSCSI RFC violations fixes. If you are an IET user and consider IET problemless, I'm sorry for the bad news. IET works well only on "fast" paths and regularly used branches, in many other less used cases IET has various problems, from simply ignoring error processing, as it is with memory allocations, to crashing itself with BUG() macro, as it is for malformed packets from initiators. ChangeLog file lists most noticeable fixes, but there were a lot of many other smaller ones.

• Due to its reworked IO architecture and SCST backend iSCSI-SCST has better performance in many cases. In future with upcoming SCST improvements, like zero-copy with Linux cache FILEIO, the performance difference is going to be even bigger. Currently in tests from a single initiator over a single connection on 1GbE hardware over FILEIO vdisk iSCSI-SCST with default settings usually outperforms tuned IET a on 3-10% (by default IET has not too good settings, so it shows considerably worse results) or has the similar performance. The bigger difference is expected with 10GbE hardware or with higher number of initiators, since iSCSI-SCST has less commands processing overhead per command, hence has smaller processing latency and puts less load on CPU. If you find a case where iSCSI-SCST has worse performance, than IET, please report it to SCST mailing list, it will be greatly appreciated.

Also, in contrast to IET, iSCSI-SCST is open for any new development, modifications and improvements, so people who want to fix or implement something new will not have to keep and maintain separate patches as it is currently necessary with IET. ISCSI-SCST is going to be actively developed and gain in the future new features like support for multiple connections per session, error recovery levels >0, etc., which eventually make it really "enterprise".

ISCSI-SCST is available for download from the SCST SVN repository. See the main page how to setup access to it.

If you are an IET user before installation carefully read README files of both iSCSI-SCST and the SCST core. Especially pay attention that now the LUN information for iSCSI-SCST is configured not using iscsi-scstd.conf file in /etc, but using corresponding SCST facilities. This is because now the responsibilities are divided (as it should be) between the target driver (iSCSI-SCST) and the SCST core: target driver is responsible for handling targets and their parameters, SCST core is responsible for handling backstorage.

This is target driver for QLogic 22xx/23xx Fibre Channel cards, but 24xx and later cards are not supported by it.

The latest version is 0.9.5. Requires Linux kernel version 2.6.16.x or higher and SCST version 0.9.5 or higher. Tested mostly on i386 and x86_64, but should work on any other supported by Linux platform.

You can find the latest development version of this driver in the SCST SVN. See the main page how to setup access to it.

Download released versions
Change Log