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210 lines
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210 lines
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<title>SCST: A Generic SCSI Target Subsystem 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">Generic SCSI Target Subsystem for Linux</h2>
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<li id="current"><a href="index.html">Home</a></li>
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<li><a href="https://github.com/SCST-project/scst">Main</a></li>
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<li><a href="https://github.com/SCST-project/scst/releases">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="users.html">Users</a></li>
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<h1>Generic SCSI Target Subsystem for Linux</h1>
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<p>The generic SCSI target subsystem for Linux (SCST) allows creation of sophisticated storage devices
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from any Linux box. Those
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devices can provide advanced
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functionality, like replication, thin provisioning,
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deduplication, high availability,
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automatic backup, etc. Another class of such devices
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are Virtual Tape Libraries (VTL)
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as well as other disk-based backup solutions.</p>
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<p>SCST devices can use any link which supports
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SCSI-style data exchange: <strong>iSCSI</strong>, <strong>Fibre Channel</strong>, <strong>FCoE</strong>,
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<strong>SAS</strong>,
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<strong>InfiniBand (SRP)</strong>, <strong>Wide (parallel) SCSI</strong>, etc.</p>
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<p>It might
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well be that your favorite storage appliance is running SCST in the firmware.</p>
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<p>SCST project consists from a set of subprojects: generic SCSI target mid-layer itself (SCST core)
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with a set of device handlers as well as target drivers
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and user space utilities.
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<h1>Features of SCST Core</h1>
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<ul>
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<li><span>SCST core 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.</span></li>
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<li><span>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 Marvell
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SAS adapters or for InfiniBand SRP are less 3000 lines of code long.</span></li>
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<li><span>Very low overhead and fine-grained locks allow to reach the
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maximum 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 architecture allows various I/O
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modes in backstorage handling. For example, pass-through device handlers allow to export real
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SCSI hardware and vdisk device handler allows to export 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:</span>
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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 SCSI standards.</span></li>
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</ul>
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</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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<p>You can find comparison of SCST with other SCSI targets on the <a href="comparison.html">Comparison</a> page.
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Some highlights what it can mean for end users you can find on the <a href="target_iscsi.html">iSCSI-SCST</a> page.
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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>Pass-through mode 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: 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). 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>FILEIO mode, 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 cache.</span></li>
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<li><span>BLOCKIO mode, 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>User space mode using scst_user device handler, which allows to implement in the
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user space high performance virtual SCSI devices. Comparing with fully in-kernel dev handlers
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this mode has very low overhead (few %%).</span></li>
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<li><span>Performance testing device handlers as well as NULLIO mode to provide
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a way for direct performance measurements without overhead of actual data
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transfers from/to underlying SCSI devices.
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</span></li>
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</ul>
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<h1>Certification</h1>
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<p>SCST core engine has passed VMware certification as part of
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<a href="http://scalecomputing.com/products">Scale’s Intelligent Clustered Storage</a> technology
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and <a href="http://www.iss-integration.com/supercore.html">ISS STORCIUM</a> solution. It also has passed
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VMware and Microsoft certification as part of <a href="https://www.sandisk.com/business/datacenter/products/flash-software/ion-accelerator">SanDisk ION Accelerator</a>
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and storage arrays developed by <a href="http://www.open-e.com/">Open-E, Inc</a>.</p>
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<p>In October 2012 Hewlett-Packard ProLiant BL465c Gen8 witrh SCST-based storage earned the maximum
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score 59.99@62 tiles in <a href="http://www.vmware.com/a/vmmark">VMmark Version 2.1.1</a>.
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In June 2016 this result was updated by <a href="http://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/vmmark/2016-06-07-HPE-ProLiant-DL580G9.pdf">HPE ProLiant DL580 Gen9</a></p>
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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="https://events.static.linuxfound.org/sites/events/files/slides/lcna15_bvanassche.pdf">SCST overview slides</a></p>
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<p><a href="http://monklinux.blogspot.com/2012/02/scst-configuration-how-to-using-gentoo.html">Gentoo HOWTO</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="SCST_Gentoo_HOWTO.txt">Gentoo 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="sgv_cache.html">SCST SGV Cache Description</a></p>
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<p><a href="http://scst.sourceforge.net/scst_user_spec.pdf">SCST user space device handler interface description</a></p>
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<h1>Articles</h1>
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<p>By Marc Smith:</p>
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<p><a href="http://marcitland.blogspot.com/2011/03/accelerating-vdi-using-scst-and-ssds.html">Accelerating VDI Using SCST and SSDs</a></p>
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<p><a href="http://marcitland.blogspot.com/2013/04/building-using-highly-available-esos.html">Building & Using a Highly Available ESOS Disk Array</a></p>
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<p><a href="http://marcitland.blogspot.com/2014/07/open-storage-dual-controller-oss-disk.html">Open Storage: Dual-Controller OSS Disk Array</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>all 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="http://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> </h1>
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<p><a href="http://sourceforge.net/donate/index.php?group_id=110471">
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<img src="http://images.sourceforge.net/images/project-support.jpg" width="88" height="32" border="0" alt="Support This Project"> </a></p>
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