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d257a9541d
- Fileio_tgt updated to be able to use those neww facilities - Docs updated. Particulary, a new dociment describing the SGV cache added git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@986 d57e44dd-8a1f-0410-8b47-8ef2f437770f
184 lines
11 KiB
HTML
184 lines
11 KiB
HTML
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<title>SCST: A Generic SCSI Target Subsystem for Linux</title>
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<div id="wrap">
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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>Generic SCSI Target Subsystem for Linux</h2>
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<ul>
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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/News</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="comparison.html">Comparison</a></li>
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<li><a href="solutions.html">Solutions</a></li>
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<div id="content-wrap">
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<div id="main">
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<h1>Generic SCSI Target Subsystem for Linux</h1>
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<p>The <strong>generic SCSI target subsystem for Linux (SCST)</strong> is an alternative
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implementation of a SCSI target subsystem for Linux. It provides unified,
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consistent interface between SCSI target drivers and
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Linux kernel as well as between Linux kernel and storage backend
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handlers, connecting target drivers with real or emulated storage backend.
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<p>SCST allows creation of sophisticated storage devices, which provide advanced
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functionality, like <strong>replication</strong>, <strong>thin provisioning</strong>,
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<strong>deduplication</strong>, <strong>high availability</strong>,
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<strong>automatic backup</strong>, etc. Another class of such devices
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are <strong>Virtual Tape Libraries</strong> (VTL)
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as well as other disk-based backup solutions. SCST created devices not
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limited by IP networking only. They can use any link which supports
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SCSI-style data exchange, including <strong>Fibre Channel</strong>,
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<strong>iSCSI</strong>, <strong>SAS</strong>,
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<strong>InfiniBand</strong> and <strong>parallel (Wide) SCSI</strong>. It might
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well be that your favorite storage appliance running SCST in the firmware.</p>
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<p>SCST project consists from a set of subprojects: <strong>generic SCSI target mid-layer itself (SCST core)</strong>
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with a set of <strong>device handlers</strong> as well as <strong>target drivers</strong>
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and <strong>user space utilities</strong>.
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<h1>Features of SCST Core</h1>
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<ul>
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<li><span>SCST core has simple, easy to use interface with target drivers.
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SCST core performs all 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>SCST core 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, target drivers for QLogic
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22xx/23xx adapters and for InfiniBand SRP are only about 2300 lines of code long.</span></li>
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<li><span>Very low overhead, fine-grained locks and simplest commands processing path 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 without any
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extra context switches.</span></li>
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<li><span>Device handlers, i.e. plugins, architecture allows 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>Advanced per-initiator devices visibility management (LUN masking) 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.
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This feature is required for hardware targets, which don't have ability to create
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virtual targets (SAS adapters, for instance).</span></li>
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<li><span>SCST core emulates necessary functionality of SCSI host adapter, because from remote initiators' point of view
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a SCSI target 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>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>SCST core has multithreaded design and complete SMP support, so, if necessary, all your CPU cores 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 remote 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, i.e. remote initiators
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will not know about it, 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 passed through SCST core.</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>. In this mode you can, for instance, share your parallel SCSI tape or SATA
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DVD-RW device to your iSCSI network.</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 Linux <strong>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 well 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> as well as <strong>NULLIO</strong> mode provide
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a way for direct performance measurements without overhead of actual data
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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="iscsi-scst-howto.txt">HOWTO For iSCSI-SCST</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="http://lpfcxxxx.sourceforge.net/HOWTO.lpfc">HOWTO For Emulex lpfc 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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<p><a href="sgv_cache.txt">SCST SGV Cache 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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