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151 lines
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HTML
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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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<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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<li id="sponsorship"><a href="sponsorship.html">Sponsorship</a></li>
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<li id="current"><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><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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<h1>Generic SCSI Target Middle Level for Linux</h1>
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<p>The <strong>SCSI target mid-level subsystem for Linux (SCST)</strong> is a subsystem of the Linux kernel
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that provides a standard framework for SCSI target drivers development.
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It is designed to provide unified, consistent interface between SCSI target drivers and Linux kernel
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and simplify target drivers development as much as possible.</p>
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<p>A system with a SCSI target device is able to share its local or virtual devices with other systems
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on a network with SCSI protocol support, e.g. <strong>SCSI bus, SAS, Fibre Channel or iSCSI</strong>.
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This is commonly used for data storage virtualization.</p>
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<p>This project consists from a set of subprojects: <strong>SCST core</strong> itself
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as well as <strong>target drivers</strong> and <strong>user space utilities</strong>.
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They are developed more or less independently and have own maintainers.</p>
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<h1>Features of SCST Core</h1>
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<ul>
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<li><span>Simple, easy to use interface with target drivers.
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Particularly, SCST core performs required pre- and post- processing of incoming requests as well as
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necessary error recovery.</span></li>
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<li><span>Undertakes most problems, related to execution contexts, thus practically eliminating one of the most
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complicated problem in the kernel drivers development. For example, a target driver for QLogic
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22xx/23xx cards, which has all necessary features, is only about 2000 lines of code long.</span></li>
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<li><span>Very low overhead, fine-grained locks and simplest commands processing path, which allow to reach
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maximum possible performance and scalability. Particularly, incoming requests can be processed in
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the caller's context or in one of the internal SCST core's tasklets, therefore no extra context switches
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required. </span></li>
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<li><span>Device handlers, i.e. plugins, architecture provides extra flexibility by allowing various I/O
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modes in backstorage handling. For example, pass-through device handlers allows to use real
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SCSI hardware and vdisk device handler allows to use files as virtual disks.</span></li>
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<li><span>Provides advanced per-initiator device visibility management (LUN masking), which allows different
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initiators to see different set of devices with different access permissions. For instance,
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initiator A could see exported from target T devices X and Y read-writable, and initiator B from
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the same target T could see devices Y read-only and Z read-writable.</span></li>
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<li><span>Emulates necessary functionality of SCSI host adapter, because from remote initiators point of view
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SCST acts as a SCSI host with its own devices. This is especially important in pass-through mode with
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one to many relationship, i.e. when multiple initiators can connect to the exported pass-through
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devices. You can find more deep elaboration why it is needed in <a href=http://www.mail-archive.com/linux-scsi@vger.kernel.org/msg06911.html>this</a>
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message in thread "Question for pass-through target design" in linux-scsi mailing list. Some of the emulated functions are the following:
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<ul>
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<li><span>Generation of necessary UNIT ATTENTIONs, their storage and delivery to all connected
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remote initiators.</span></li>
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<li><span>RESERVE/RELEASE functionality.</span></li>
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<li><span>CA/ACA conditions (not implemented yet).</span></li>
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<li><span>All types of RESETs and other task management functions.</span></li>
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<li><span>REPORT LUNS command as well as SCSI address space management in order to have consistent
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address space on all remote initiators, since local SCSI devices could not know about each
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other to report via REPORT LUNS command. Additionally, SCST core responds with error on all
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commands to non-existing devices and provides access control, so different remote
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initiators could see different set of devices.</span></li>
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<li><span>Other necessary functionality (task attributes, etc.) as specified in SAM-2, SPC-2, SAM-3,
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SPC-3 and other SCSI standards.</span></li>
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</ul>
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</span></li>
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<li><span>Multithreaded design and complete SMP support, so, if necessary, all your processors will participate in the commands
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processing.</span></li>
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<li><span>Well documented.</span></li>
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</ul>
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<p>Interoperability between SCST core and local SCSI initiators (i.e. sd, st, etc.) is the additional issue that SCST is going to
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address (it is not implemented yet). It is necessary, because local SCSI initiators can change the state of the
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device, for example RESERVE the device, or some of its parameters and that could be done behind SCST, which could
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lead to various problems, including data corruption. Thus, RESERVE/RELEASE commands, locally generated
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UNIT ATTENTIONs, etc. should be intercepted and processed as if local SCSI initiators act as remote SCSI
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initiators connected to SCST.</p>
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<p>Interface between SCST core and target drivers is based on work, done by <a href="http://www.iol.unh.edu/">University
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of New Hampshire Interoperability Labs (UNH IOL)</a> for the <a href="http://www.iol.unh.edu/consortiums/iscsi/index.html">
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UNH-iSCSI project</a>, which was developed on <a href="http://unh-iscsi.sourceforge.net/">SourceForge.net</a>.</p>
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<h1>SCST core supports the following I/O modes</h1>
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<ul>
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<li><span><strong>Pass-through mode</strong> with one to many relationship, i.e. when multiple initiators can
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connect to the exported pass-through devices, for virtually all SCSI devices types: <strong>disks (type 0),
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tapes (type 1), processors (type 3), CDROMs (type 5), MO disks (type 7), medium changers (type 8) and RAID
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controllers (type 0xC)</strong>.</span></li>
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<li><span><strong>FILEIO mode</strong>, which allows to use files on file systems or block devices as virtual
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remotely available SCSI disks or CDROMs with benefits of the <strong>Linux page cache</strong>.</span></li>
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<li><span><strong>BLOCKIO mode</strong>, which performs direct block IO with a block device, bypassing
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page-cache for all operations. This mode works ideally with high-end storage HBAs and for applications that
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either do not need caching between application and disk or need the large block throughput.</span></li>
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<li><span><strong>User space mode</strong> using scst_user device handler, which allows to implement in the
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user space virtual SCSI devices in the SCST environment.</span></li>
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<li><span><strong>"Performance" device handlers</strong>, which provide in pseudo pass-through mode a way for
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direct performance measurements without overhead of actual data transferring from/to underlying SCSI device.
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</span></li>
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</ul>
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</div>
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<div id="rightbar">
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<h1>Documentation</h1>
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<p><a href="scst_pg.html">HTML</a></p>
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<p><a href="scst_pg.pdf">PDF</a></p>
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<p><a href="qla2x00t-howto.html">HOWTO For QLogic Target Driver</a></p>
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<p><a href="scst_user_spec.txt">SCST User Interface Description</a></p>
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<h1>SCST 0.9.6 graphs</h1>
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<p><a href=images/init_scst.png>init_scst</a></p>
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<p><a href=images/scst_cmd_thread.png>scst_cmd_thread</a></p>
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<p><a href=images/scst_mgmt_cmd_thread.png>scst_mgmt_cmd_thread</a></p>
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<p><a href=images/scst_mgmt_thread.png>scst_mgmt_thread</a></p>
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<p>by Ming Zhang</p>
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<h1>QUESTIONS</h1>
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<p>If you have any questions you can ask them via<br><a href=https://lists.sourceforge.net/lists/listinfo/scst-devel>
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scst-devel mailing list</a><br><br>
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See <a href=http://sourceforge.net/mail/?group_id=110471>mailing lists page</a> for more info about SCST mailing
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lists.</p>
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<h1>SourceForge</h1>
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<p><a href="http://sourceforge.net">
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<img src="http://sourceforge.net/sflogo.php?group_id=110471&type=2" alt="SourceForge.net Logo" border="0">
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</a></p>
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© Copyright 2008 <b><font color="#EC981F">Vladislav Bolkhovitin & others.</font>
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