Re: iSCSI: Out Of Sequence due to null sequence with multiple connections.

[Sandeep Joshi <sandeepj@research.bell-labs.com>]

Black_David@emc.com wrote:
> 
> Sandeep,
> 
> > Still wading thru related emails but I believe that if a refCmdSN
> > is added to the task management PDU (not present currently but
> > could be added for task-related management commands), then it
> > might fix the above-mentioned flaws and allow for safe execution
> > and immediate delivery of the abort task to the target.
> 
> I think this is a functional subset of Doug Otis's suggestion to always
> use a CmdSN and add a header flag indicating that the command should
> be executed immediately at the target rather than waiting for those with
> prior CmdSNs to arrive.  Doug's suggestion also consumes less space
> in the header. 

Yes, i now understand Doug's proposal.  The "PDU sequencer" was
new terminology which baffled me some.. :-)

His solution does seem like a cleaner approach (no target state)

In some ways, it reminds one of the M_FLUSH function used
in the STREAMS layer.   If we stand back a bit, we see that the
scenario is essentially the same -> Stream module processing is 
normally asynchronous.   With M_FLUSH, the stream head desires to 
clean out all the downstream module queues.  

The only concern with that solution is that it may delete
a whole lot of unrelated SCSI I/O tasks as the sequencer cmdSN
moves way up to the cmdSN of the task_mgmt command.  "Abort task" 
as such will not exist in the iSCSI world.   These unrelated 
commands will now have to be retried by the initiator.  

Do we have any numbers on how often ABORT_TASK/ABORT_TASK_SET actually 
occurs in filesystem/SCSI workloads ?

regards,
-Sandeep