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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