Question about SCSI consistency model
Vladislav Bolkhovitin
vst at vlnb.net
Thu Oct 24 10:22:32 PDT 2013
* From the T10 Reflector (t10 at t10.org), posted by:
* Vladislav Bolkhovitin <vst at vlnb.net>
*
Hello,
We are working on creating a distributed SCSI device, when several nodes
are combined together to create something, which looks as a single
multipath SCSI device to initiators, where each path is the path to
separate node. We figured out that fully exploiting SCSI consistency
model would allow us to significantly improve performance, but we wonder
if the SCSI consistency model is going AS far as we need.
SPC-3 section "Write and unmap failures" says:
_____________________________________________________________________________
_____
If one or more write commands are have not completed when a power loss
occurs (e.g., resulting in a vendor specific command timeout by the
application client) or a medium error or hardware error occurs (e.g.,
because a removable medium was incorrectly undemounted), then any data
in the logical blocks referenced by the LBAs specified by any of those
commands is indeterminate. Before sending a read command or verify
command specifying any LBAs that were specified by one of the write
commands that did not complete, the application client should resend
that write command. If an application client sends a read command or
verify command specifying any LBAs that were specified by one of the
write commands that did not complete before resending that write
command, then the device server may return old data, new data,
vendor-specific data, or any combination thereof for the logical blocks
referenced by the specified LBAs
_____________________________________________________________________________
_____
The question is if the device server after a failure of a write command
on block X starts returning on reads from this block from one path - old
data and from another path - new data, would it still be in line with
the above SCSI consistency model?
Thanks,
Vlad
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