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<div id="main">
<h1>ISCSI target driver iSCSI-SCST</h1>
<p><strong>ISCSI-SCST</strong> is a forked (with all respects) version of <strong>IET</strong> with updates to work
over <strong>SCST</strong> as well as with many improvements and bugfixes. The reason of fork is that the necessary
changes are intrusive and with the current <strong>IET</strong> 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
<strong>IET</strong> trunk.</p>
<p><strong>ISCSI-SCST</strong> has the following major advantages over <strong>IET</strong>. They are summarized on the
<a href="comparison.html">Comparison</a> page.</p>
<p><strong>ISCSI-SCST</strong> is a forked (with all respects) version of IET. Reasons of the fork were:</p>
<ul>
<li><font color="#666666">ISCSI-SCST uses full power of <strong>SCST core</strong>, hence has the following additional features:</font>
<ul>
<li><span><strong>Pass-through mode with one to many relationship</strong>, i.e. when multiple initiators can connect to
exported pass-through devices. For instance, in this mode you can safely export your parallel
<strong>SCSI tape</strong> or <strong>tape library</strong> on
your iSCSI net and multiple initiators can share it without risk of data loss because of the
shared usage. Existing outdated "rawio" patch for IET supports only non-enforced 1:1
relationship, so it is unsafe to use it in multiple initiators environments.</span></li>
<li><span>To be able to use full power of <strong>SCST core</strong>.</span></li>
<li><span>To fix all the problems, corner cases issues and iSCSI standard violations IET which has.</span></li>
</ul>
<li><span><strong>More advanced devices visibility management</strong>, when different initiators can see different set of
devices with different access permissions from the same target.</span></li>
<p>IET is a well recognized and widely used iSCSI target, but, frankly speaking, it works more or less well
on fast paths and regularly used code branches only, however in many corner
cases it has a lot of problems, like ignoring error processing, as it is for memory allocations,
crashing itself with BUG() macro, as it is for malformed packets from initiators, possible data
corruptions, because of, for instance, unsafe task management or sessions reinstatement implementations, etc.
There was no way to fix all them without a fork. As the result of this effort nearly 90% of the IET kernel code was fully
rewritten.</p>
<p>Since ISCSI-SCST uses SCST core, it has the following additional features:</font></p>
<li><span><strong>O_DIRECT</strong>, i.e. <strong>"BLOCKIO on files"</strong>, 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.</span></li>
<ul>
<li><span><strong>Pass-through mode with one to many relationship</strong>, i.e. when multiple initiators can connect to
exported pass-through devices. For instance, in this mode you can safely export your parallel
<strong>SCSI tape</strong> or <strong>tape library</strong> (or <strong>VTL</strong>) on
your iSCSI net and multiple initiators can share it without risk of data loss because of the
shared usage. Existing outdated "rawio" patch for IET supports only non-enforced 1:1
relationship, so it is unsafe to use it in multiple initiators environments.</span></li>
<li><span><strong>4KB blocks</strong> eliminate abysmal write performance caused by misaligned partitions.
</span></li>
<li><span><strong>More advanced devices visibility management</strong>, when different initiators can see different set of
devices with different access permissions from the same target.</span></li>
<li><span><strong>Virtual CD/DVD-ROMs</strong>.</span></li>
<li><span><strong>O_DIRECT</strong>, i.e. <strong>"BLOCKIO on files"</strong>, 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.</span></li>
<li><span>Ability to create target devices emulators in the <strong>user space</strong>.</span></li>
<li><span><strong>4KB blocks</strong> eliminate abysmal write performance caused by misaligned partitions.
</span></li>
<li><span>Ability to create <strong>multi-transport SCSI targets</strong>, which can export (possibly, the same)
devices over multiple transports.</span></li>
</ul>
</li>
<li><span><strong>Virtual CD/DVD-ROMs</strong>.</span></li>
<li><span>ISCSI-SCST has many code improvements and cleanups, including stability
and iSCSI RFC violations fixes.
IET works well on "fast" paths and regularly used code branches, but in many corner
cases it has various problems, from simply ignoring error processing, as it is for memory
allocations, and crashing itself with BUG() macro, as it is for malformed packets from initiators, to possible data
corruptions, because of, for instance, unsafe task management or sessions reinstatement implementations.</span></li>
<li><span>Ability to create target devices emulators in the <strong>user space</strong>.</span></li>
<li><span>Due to reworked I/O architecture and SCST backend iSCSI-SCST has much
better performance in many cases and has potential for future improvements, like zero-copy
with Linux cache FILEIO. In many tests iSCSI-SCST outperforms tuned for best
performance IET on more than 100%.</span></li>
</ul>
<li><span>Ability to create <strong>multi-transport SCSI targets</strong>, which can export (possibly, the same)
devices over multiple transports.</span></li>
</ul>
<p>Also, due to reworked I/O architecture and SCST backend iSCSI-SCST has much
better performance in many cases and has potential for future improvements, like zero-copy
with Linux cache FILEIO. In many tests iSCSI-SCST outperforms tuned for best
performance IET on more than 100%.</p>
<p>Advantages of iSCSI-SCST over IET are summarized on the <a href="comparison.html">Comparison</a> page.</p>
<p>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