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scst/README: Add a diagram that illustrates an active/non-optimized setup

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git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@7967 d57e44dd-8a1f-0410-8b47-8ef2f437770f
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Bart Van Assche
2019-02-19 01:27:49 +00:00
parent 50d93363e2
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1 changed files with 39 additions and 12 deletions
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@@ -1948,23 +1948,50 @@ information about ALUA support in Windows Server, see also:
Active/Non-Optimized via internal redirection
.............................................
The Active-Standby configuration is simple to understand and setup,
however, it might have serious interoperability issues, because not all
initiators handle Standby state correctly. For instance, some versions
of VMware reported to have such issues. Same for Windows.
The Active-Standby configuration is simple to understand and to set up.
However, it might cause serious interoperability issues because not all
initiators handle the ALUA state 'standby' state correctly. For instance,
some versions of VMware reported to have such issues. Same for Windows.
Hence, it is better to use Non-Optimized state on the passive node
instead of Standby with internal commands redirection to the active
node. This is what the vast majority of storage vendors are doing, which
is, actually, the reason why Standby and Unavailable states have all
those initiator issues. Simply, they have had too few testing, because
only marginally used.
It is better to use the 'nonoptimized' state on the passive node instead
of 'standby' with internal commands redirection to the active node. This
is what the vast majority of storage vendors are doing. This is actually
the reason why the 'standby' and 'unavailable' states have all those
initiator interoperability issues. The latter combination has received
too few testing because it is only marginally used.
SCST has necessary support for such redirection, it just needs to be
SCST has the necessary support for such redirection, it just needs to be
configured correctly. It's a little bit of effort, especially to
understand how it's going to function, but then it would work MUCH more
reliable for full range of initiators. Ever poor initiators, who have no
idea about ALUA (boot from SAN, e.g.) would work now.
idea about ALUA (boot from SAN, e.g.) would work now. The following
diagram illustrates this approach:
................................................................
. . .
. Initiator A . Initiator B .
. | . | .
................................................................
. | . | .
. target port C . target port D .
. | . | .
. SCST . SCST .
. Instance E - target . target - Instance F .
. / \ port G . port H / \ .
. / \ \./ / \ .
. / \ /.\ / \ .
. vdisk_blockio dev_disk / . \ dev_disk vdisk_blockio .
. handler handler / . \ handler handler .
. | | / . \ | | .
. block device SCSI / . SCSI block device .
. I initiator . initiator J .
. | node K . node L | .
. |______________________ .______________________| .
................................................................
The link between block devices I and J stands for synchronous replication.
Such a setup can be configured as follows:
1. Build SCST with CONFIG_SCST_FORWARD_MODE_PASS_THROUGH enabled in scst.h