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9 <title>SCST: Generic SCSI Target Middle Level for Linux</title>
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15 <div class="logoimg"></div><h1 id="logo"><span class="orange"></span></h1>
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16 <h2 id="slogan">SCSI Target Middle Level for Linux</h2>
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31 <h1>Generic SCSI Target Middle Level for Linux</h1>
33 <p>The <strong>SCSI target mid-level subsystem for Linux (SCST)</strong> is a subsystem of the Linux kernel
34 that provides a standard framework for SCSI target drivers development.
35 It is designed to provide unified, consistent interface between SCSI target drivers and Linux kernel
36 and simplify target drivers development as much as possible.</p>
37 <p>A system with a SCSI target device is able to share its local or virtual devices with other systems
38 on a network with SCSI protocol support, e.g. <strong>SCSI bus, SAS, Fibre Channel or iSCSI</strong>.
39 This is commonly used for data storage virtualization.</p>
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41 <p>This project consists from a set of subprojects: <strong>SCST core</strong> itself
42 as well as <strong>target drivers</strong> and <strong>user space utilities</strong>.
43 They are developed more or less independently and have own maintainers.</p>
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45 <h1>Features of SCST Core</h1>
47 <li><span>Simple, easy to use interface with target drivers.
48 Particularly, SCST core performs required pre- and post- processing of incoming requests as well as
49 necessary error recovery.</span></li>
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50 <li><span>Undertakes most problems, related to execution contexts, thus practically eliminating one of the most
51 complicated problem in the kernel drivers development. For example, a target driver for QLogic
52 22xx/23xx cards, which has all necessary features, is only about 2000 lines of code long.</span></li>
53 <li><span>Very low overhead, fine-grained locks and simplest commands processing path, which allow to reach
54 maximum possible performance and scalability. Particularly, incoming requests can be processed in
55 the caller's context or in one of the internal SCST core's tasklets, therefore no extra context switches
56 required. </span></li>
57 <li><span>Device handlers, i.e. plugins, architecture provides extra flexibility by allowing various I/O
58 modes in backstorage handling. For example, pass-through device handlers allows to use real
59 SCSI hardware and vdisk device handler allows to use files as virtual disks.</span></li>
60 <li><span>Provides advanced per-initiator device visibility management (LUN masking), which allows different
61 initiators to see different set of devices with different access permissions. For instance,
62 initiator A could see exported from target T devices X and Y read-writable, and initiator B from
63 the same target T could see devices Y read-only and Z read-writable.</span></li>
64 <li><span>Emulates necessary functionality of SCSI host adapter, because from remote initiators point of view
65 SCST acts as a SCSI host with its own devices. This is especially important in pass-through mode with
66 one to many relationship, i.e. when multiple initiators can connect to the exported pass-through
67 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>
68 message in thread "Question for pass-through target design" in linux-scsi mailing list. Some of the emulated functions are the following:
70 <li><span>Generation of necessary UNIT ATTENTIONs, their storage and delivery to all connected
71 remote initiators.</span></li>
72 <li><span>RESERVE/RELEASE functionality.</span></li>
73 <li><span>CA/ACA conditions (not implemented yet).</span></li>
74 <li><span>All types of RESETs and other task management functions.</span></li>
75 <li><span>REPORT LUNS command as well as SCSI address space management in order to have consistent
76 address space on all remote initiators, since local SCSI devices could not know about each
77 other to report via REPORT LUNS command. Additionally, SCST core responds with error on all
78 commands to non-existing devices and provides access control, so different remote
79 initiators could see different set of devices.</span></li>
80 <li><span>Other necessary functionality (task attributes, etc.) as specified in SAM-2, SPC-2, SAM-3,
81 SPC-3 and other SCSI standards.</span></li>
84 <li><span>Multithreaded design and complete SMP support, so, if necessary, all your processors will participate in the commands
85 processing.</span></li>
86 <li><span>Well documented.</span></li>
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88 <p>Interoperability between SCST core and local SCSI initiators (i.e. sd, st, etc.) is the additional issue that SCST is going to
89 address (it is not implemented yet). It is necessary, because local SCSI initiators can change the state of the
90 device, for example RESERVE the device, or some of its parameters and that could be done behind SCST, which could
91 lead to various problems, including data corruption. Thus, RESERVE/RELEASE commands, locally generated
92 UNIT ATTENTIONs, etc. should be intercepted and processed as if local SCSI initiators act as remote SCSI
93 initiators connected to SCST.</p>
94 <p>Interface between SCST core and target drivers is based on work, done by <a href="http://www.iol.unh.edu/">University
95 of New Hampshire Interoperability Labs (UNH IOL)</a> for the <a href="http://www.iol.unh.edu/consortiums/iscsi/index.html">
96 UNH-iSCSI project</a>, which was developed on <a href="http://unh-iscsi.sourceforge.net/">SourceForge.net</a>.</p>
98 <h1>SCST core supports the following I/O modes</h1>
100 <li><span><strong>Pass-through mode</strong> with one to many relationship, i.e. when multiple initiators can
101 connect to the exported pass-through devices, for virtually all SCSI devices types: <strong>disks (type 0),
102 tapes (type 1), processors (type 3), CDROMs (type 5), MO disks (type 7), medium changers (type 8) and RAID
103 controllers (type 0xC)</strong>.</span></li>
104 <li><span><strong>FILEIO mode</strong>, which allows to use files on file systems or block devices as virtual
105 remotely available SCSI disks or CDROMs with benefits of the <strong>Linux page cache</strong>.</span></li>
106 <li><span><strong>BLOCKIO mode</strong>, which performs direct block IO with a block device, bypassing
107 page-cache for all operations. This mode works ideally with high-end storage HBAs and for applications that
108 either do not need caching between application and disk or need the large block throughput.</span></li>
109 <li><span><strong>User space mode</strong> using scst_user device handler, which allows to implement in the
110 user space virtual SCSI devices in the SCST environment.</span></li>
111 <li><span><strong>"Performance" device handlers</strong>, which provide in pseudo pass-through mode a way for
112 direct performance measurements without overhead of actual data transferring from/to underlying SCSI device.
116 <div id="rightbar">
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117 <h1>Documentation</h1>
118 <p><a href="scst_pg.html">HTML</a></p>
119 <p><a href="scst_pg.pdf">PDF</a></p>
120 <p><a href="iscsi-scst-howto.txt">HOWTO For iSCSI-SCST</a></p>
121 <p><a href="qla2x00t-howto.html">HOWTO For QLogic Target Driver</a></p>
122 <p><a href="scst_user_spec.txt">SCST User Interface Description</a></p>
123 <h1>SCST 0.9.6 graphs</h1>
124 <p><a href=images/init_scst.png>init_scst</a></p>
125 <p><a href=images/scst_cmd_thread.png>scst_cmd_thread</a></p>
126 <p><a href=images/scst_mgmt_cmd_thread.png>scst_mgmt_cmd_thread</a></p>
127 <p><a href=images/scst_mgmt_thread.png>scst_mgmt_thread</a></p>
130 <p>If you have any questions you can ask them via<br><a href=https://lists.sourceforge.net/lists/listinfo/scst-devel>
131 scst-devel mailing list</a><br><br>
132 See <a href=http://sourceforge.net/mail/?group_id=110471>mailing lists page</a> for more info about SCST mailing
134 <h1>SourceForge</h1>
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