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33                                 <h1>Target Drivers</h1>\r
34                                 <ul class="sidemenu">
35                                         <li><a href="target_iscsi.html">iSCSI-SCST</a></li>
36                                         <li><a href="target_qla22xx_23xx.html">QLogic FC 22xx/23xx</a></li>
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50                                 <h1>ISCSI target driver iSCSI-SCST</h1>
51                                 <p><strong>ISCSI-SCST</strong> is a forked (with all respects) version of <strong>IET</strong> with updates to work 
52                                 over <strong>SCST</strong> as well as with many improvements and bugfixes. The reason of fork is that the necessary 
53                                 changes are intrusive and with the current <strong>IET</strong> merge policy, where only simple bugfix-like patches,
54                                  which doesn't touch the core code, could be merged, it is very unlikely that they will be merged in the main 
55                                  <strong>IET</strong> trunk.</p>
57                                 <p><strong>ISCSI-SCST</strong> has the following major advantages over the mainline <strong>IET</strong>:
58                                 <ul>
59                                         <li><span>It uses full power of <strong>SCST core</strong> without loosing any existing feature (except, maybe, illegal
60                                          from SCSI specifications point of view MPIO). Namely, you can additionally use with it:
61                                          <ul>
62                                                 <li><span><strong>Pass-through mode with one to many relationship</strong>, i.e. when multiple initiators can connect to the 
63                                                 exported pass-through devices. For instance, you can safely export your parallel
64                                                 <strong>SCSI tape</strong> or <strong>tape library</strong> on 
65                                                 your iSCSI net and multiple initiators can share it without risk of data loss because of the 
66                                                 shared usage. Existing "rawio" patch for IET supports only non-enforced 1:1 
67                                                 relationship, so it is unsafe to use it in multiple initiators environments.</span></li>
68                                                 <li><span><strong>More advances devices visibility management</strong>, when different initiators can see different set of 
69                                                 devices with different access permissions from the same target.</span></li>
70                                                 <li><span><strong>O_DIRECT</strong>, i.e. "<strong>BLOCKIO</strong> on files", mode, which has all advantages 
71                                                 of <strong>BLOCKIO</strong>, but also supports files on file systems. Sometimes, in the appropriate cases, 
72                                                 this mode can make performance difference in 100% or even more.</span></li>
73                                                 <li><span>With <strong>4KB blocks</strong> you can forget about abysmal write performance caused by misaligned partitions. 
74                                                 All modern OS'es, including Windows starting from, at least, Windows 2000, work perfectly with 4KB block 
75                                                 devices without any additional storage or handling overhead.</span></li>
76                                                 <li><span><strong>Virtual CD/DVD-ROMs</strong> without necessity for manual patching.</span></li>       
77                                                 <li><span>Ability to create target devices emulators in the <strong>user space</strong>.</span></li>
78                                                 <li><span>Ability to create <strong>multi-transport SCSI targets</strong>, which can export (possibly, the same) 
79                                                 devices over multiple transports.</span></li>                                                                                                                            
80                                          </ul>                                   
81                                          </span></li>
82                                          <li><span>It has many code improvements and cleanups, including <strong>stability and iSCSI RFC violations fixes</strong>. 
83                                           Many IET users use it for ages without problems, so they consider it problemless. But,
84                                           in fact, unfortunately, it isn't so.
85                                           IET works well only on "fast" paths and regularly used branches, in many other less used
86                                           cases IET has various problems, from simply ignoring error processing, as it is with memory 
87                                           allocations, and crashing itself with BUG() macro, as it is for malformed packets from initiators, to possible data
88                                           corruption. See, for instance, <a href="http://communities.vmware.com/thread/53797?tstart=0&start=15">this</a>
89                                           thread on a VMware forum about in which <strong>Russian roulette</strong> IET users play using it with VMware.
90                                           ChangeLog file lists most noticeable fixes, but there were a lot of many other smaller ones.</span></li>
91                                          <li><span>Due to reworked I/O architecture and <strong>SCST</strong> backend <strong>iSCSI-SCST</strong> has
92                                           better performance in many cases. In future with upcoming improvements in <strong>SCST core</strong>, like zero-copy 
93                                           with Linux cache <strong>FILEIO</strong>, the performance difference is going to be even bigger. Currently in 
94                                           tests from a single initiator over a single connection on 1GbE hardware over <strong>FILEIO</strong> vdisk 
95                                           <strong>iSCSI-SCST</strong> with default settings usually outperforms tuned for best
96                                           performance <strong>IET</strong> a on <strong>3-30%</strong>. The difference is especially noticeably with real storage,
97                                           not NULLIO or RAM disks.
98                                           On 10GbE hardware the performance difference is often as high as <strong>100-200%</strong> or even more.
99                                           With higher number of initiators the difference will be even bigger. This is because 
100                                           <strong>iSCSI-SCST</strong> has less commands processing overhead per command, hence has smaller processing 
101                                           latency and puts less load on CPU.                    
102                                 </ul></p>\r
103                                 <p>Also, in contrast to <strong>IET, iSCSI-SCST</strong> is open for any new development, modifications and 
104                                 improvements, so people who want to fix or implement something new will not have to keep and maintain separate 
105                                 patches as it is currently necessary with <strong>IET</strong>. <strong>ISCSI-SCST</strong> is actively
106                                  developed and going to gain in the future new features like support for multiple connections per session, error recovery 
107                                  levels >0, etc., which eventually make it really <i>"Enterprise"</i>.</p>
109                                 <p>If you are an IET user before installation carefully read README files of both iSCSI-SCST and 
110                                 the SCST core. Especially pay attention that now the LUN information for iSCSI-SCST is configured not using 
111                                 iscsi-scstd.conf file in /etc, but using corresponding SCST facilities. This is because now the responsibilities 
112                                 are divided (as it should be) between the target driver (iSCSI-SCST) and the SCST core: target driver is 
113                                 responsible for handling targets and their parameters, SCST core is responsible for handling backstorage.</p>
115                                 <p class="post-footer align-right">
116                                         <a href="http://iscsitarget.sourceforge.net/" class="readmore">IET WebPage</a>                          \r
117                                         <a href="http://scst.svn.sourceforge.net/" class="readmore">SCST SVN Repository</a>     \r
118                                 </p>\r
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126                         &copy; Copyright 2008 <strong>SCST (SCSI Target Middle Level for Linux)</strong>&nbsp;&nbsp;  \r
127                         Design by: <b><font color="#EC981F">Daniel Fernandes</font></b>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;         \r
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