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e8b67bb755
- Minor cleanups git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@675 d57e44dd-8a1f-0410-8b47-8ef2f437770f
143 lines
8.8 KiB
HTML
143 lines
8.8 KiB
HTML
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<meta name="Keywords" content="Generic SCSI Target Middle Level for Linux, iSCSI, iSCSI target" />
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<meta name="author" content="Daniel Fernandes"/>
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<title>SCST: Generic SCSI Target Middle Level for Linux</title>
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<body>
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<div id="header">
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<div class="logoimg"></div><h1 id="logo"><span class="orange"></span></h1>
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<h2 id="slogan">SCSI Target Middle Level for Linux</h2>
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<div id="menu">
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<li id="sponsorship"><a href="sponsorship.html">Sponsorship</a></li>
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<li><a href="index.html">Home</a></li>
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<li><a href="http://www.sourceforge.net/projects/scst">Main</a></li>
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<li id="current"><a href="targets.html">Drivers</a></li>
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<li><a href="downloads.html">Downloads</a></li>
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<li><a href="contributing.html">Contributing</a></li>
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<li><a href="scstvsstgt.html">SCST vs STGT</a></li>
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<div id="content-wrap">
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<h1>Target Drivers</h1>
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<ul class="sidemenu">
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<li><a href="target_iscsi.html">iSCSI-SCST</a></li>
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<li><a href="target_qla22xx_23xx.html">QLogic FC 22xx/23xx</a></li>
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<li><a href="target_qla_isp.html">QLogic FC ISP</a></li>
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<li><a href="target_srp.html">SCSI RDMA Protocol (SRP)</a></li>
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<li><a href="target_lsi.html">LSI/MPT cards</a></li>
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<li><a href="target_fcoe.html">Open-FCoE Target SW</a></li>
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<li><a href="target_old.html">Old Unsupported</a></li>
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</ul>
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<h1>User utilities</h1>
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<ul class="sidemenu">
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<li><a href="scst_admin.html">SCST Admin Utility</a></li>
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<li><a href="handler_fileio_tgt.html">FILEIO_TGT handler</a></li>
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</ul>
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</div>
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<div id="main">
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<h1>ISCSI target driver iSCSI-SCST</h1>
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<p><strong>ISCSI-SCST</strong> is a forked (with all respects) version of <strong>IET</strong> with updates to work
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over <strong>SCST</strong> as well as with many improvements and bugfixes. The reason of fork is that the necessary
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changes are intrusive and with the current <strong>IET</strong> merge policy, where only simple bugfix-like patches,
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which doesn't touch the core code, could be merged, it is very unlikely that they will be merged in the main
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<strong>IET</strong> trunk.</p>
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<p><strong>ISCSI-SCST</strong> has the following major advantages over the mainline <strong>IET</strong>:
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<ul>
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<li><span>It uses full power of <strong>SCST core</strong> without loosing any existing IET feature (except, maybe, illegal
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from SCSI specifications point of view MPIO). Namely, you can additionally use with it:
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<ul>
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<li><span><strong>Pass-through mode with one to many relationship</strong>, i.e. when multiple initiators can connect to the
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exported pass-through devices. For instance, you can safely export your parallel
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<strong>SCSI tape</strong> or <strong>tape library</strong> on
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your iSCSI net and multiple initiators can share it without risk of data loss because of the
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shared usage. Existing "rawio" patch for IET supports only non-enforced 1:1
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relationship, so it is unsafe to use it in multiple initiators environments.</span></li>
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<li><span><strong>More advances devices visibility management</strong>, when different initiators can see different set of
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devices with different access permissions from the same target.</span></li>
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<li><span><strong>O_DIRECT</strong>, i.e. "<strong>BLOCKIO</strong> on files", mode, which has all advantages
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of <strong>BLOCKIO</strong>, but also supports files on file systems. Sometimes, in the appropriate cases,
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this mode can make performance difference in 100% or even more.</span></li>
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<li><span>With <strong>4KB blocks</strong> you can forget about abysmal write performance caused by misaligned partitions.
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All modern OS'es, including Windows starting from, at least, Windows 2000, work perfectly with 4KB block
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devices without any additional storage or handling overhead.</span></li>
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<li><span><strong>Virtual CD/DVD-ROMs</strong> without necessity for manual patching.</span></li>
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<li><span>Ability to create target devices emulators in the <strong>user space</strong>.</span></li>
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<li><span>Ability to create <strong>multi-transport SCSI targets</strong>, which can export (possibly, the same)
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devices over multiple transports.</span></li>
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</ul>
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</span></li>
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<li><span>It has many code improvements and cleanups, including <strong>stability and iSCSI RFC violations fixes</strong>.
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Many IET users use it for ages without problems, so they consider it free from any real problem. But,
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in fact, unfortunately, it isn't so.
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IET works well only on "fast" paths and regularly used code branches. In many other less used
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cases IET has various problems, from simply ignoring error processing, as it is with memory
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allocations, and crashing itself with BUG() macro, as it is for malformed packets from initiators, to possible data
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corruption. See, for instance, <a href="http://communities.vmware.com/thread/53797?tstart=0&start=15">this</a>
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thread on a VMware forum about in which <strong>Russian roulette</strong> IET users play using it with VMware.
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ChangeLog file lists most noticeable fixes, but there were a lot of many other smaller ones.</span></li>
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<li><span>Due to reworked I/O architecture and SCST backend iSCSI-SCST has
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better performance in many cases. In future with upcoming improvements in SCST core, like <strong>zero-copy
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with Linux cache FILEIO</strong>, the performance difference is going to be even bigger. Currently in
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tests from a single initiator over a single connection on 1GbE hardware over FILEIO vdisk
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iSCSI-SCST with default settings usually outperforms tuned for best
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performance IET a on <strong>on up to 100%</strong>. The difference is especially noticeably with real storage,
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not NULLIO or RAM disks.
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On 10GbE hardware the performance difference is often as high as <strong>100-200%</strong> or even more.
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With higher number of initiators the difference will be even bigger. This is because
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iSCSI-SCST has less commands processing overhead per command, hence has smaller processing
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latency and puts less load on CPU.
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</ul></p>
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<p>Also, in contrast to IET, iSCSI-SCST is open for any new development, modifications and
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improvements, so people who want to fix or implement something new will not have to keep and maintain separate
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patches as it is currently necessary with IET. ISCSI-SCST is actively
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developed and going to gain in the future new features like support for multiple connections per session, error recovery
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levels >0, etc., which eventually make it really <i>"Enterprise"</i>.</p>
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<p>If you are an IET user before installation carefully read README files of both iSCSI-SCST and
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the SCST core. Especially pay attention that now the LUN information for iSCSI-SCST is configured not using
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iscsi-scstd.conf file in /etc, but using corresponding SCST facilities. This is because now the responsibilities
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are divided (as it should be) between the target driver (iSCSI-SCST) and the SCST core: target driver is
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responsible for handling targets and their parameters, SCST core is responsible for handling backstorage.</p>
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<p>The latest stable version is 1.0.0. Requires Linux kernel version 2.6.16.x or higher and SCST version 1.0.0 or higher.
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Tested mostly on i386 and x86_64, but should work on any other supported by Linux platform.</p>
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<p>You can find the latest development version of this driver in the SCST SVN. See the download page how to setup
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access to it.</p>
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<p class="post-footer align-right">
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<a href="iscsi-scst-howto.txt" class="readmore">HOWTO</a>
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<a href="https://sourceforge.net/project/showfiles.php?group_id=110471&package_id=283228" class="readmore">Download</a>
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<a href="http://scst.svn.sourceforge.net/" class="readmore">SCST SVN Repository</a>
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</p>
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</div>
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</div>
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© Copyright 2008 <b><font color="#EC981F">Vladislav Bolkhovitin & others.</font>
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Design by: <b><font color="#EC981F">Daniel Fernandes</font></b>
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