Steve Byan wrote:
On Mar 8, 2006, at 12:49 PM, Vladislav Bolkhovitin wrote:
Steve Byan wrote:
I still don't understand why you are reluctant to return
TASK_SET_FULL or BUSY in this case; it's what the SCSI standard
supplies as the way to say "don't queue too many commands, please".
I don't like out of order execution, which happens practically on all
such "rejected" commands, because subsequent already queued commands
are not "rejected" with it and some of them could be accepted later.
I see, you care about order. So do tapes. The historical answer has
been to not support tagged command queuing when you care about
ordering. To dodge the performance problem due to lack of queuing, the
targets usually implement a read-ahead and write-behind cache, and then
perform queuing behind the scenes, after telling the initiator that the
command has completed. Of course, this has obvious data integrity
issues for disk-type logical units.
Yes, tapes just can't work without strict ordering. SCST was originally
done for tapes, so I still keep some kind of tape-oriented thinking :)
Actually, with current journaling file systems ordering also became more
important for disks as well. Data integrity problem in "behind the
scenes" queuing could be on practice easily solved by battery-based
backup power on the disks. In case of TASK_SET_FULL things are much
worse, because the reordering happens _between_ target and _initiator_,
since the initiator must retry "rejected" command explicitly, then in
case of the initiator crash before the command will be retried and if FS
on it uses ordering barriers to protect the integrity (Linux seems does
so, but I could be wrong), the FS data could be written out of order
with its journal and the FS could be corrupted. Even worse,
TASK_SET_FULL "rejects" basically happen every the queue length'th
command, ie very often. This is why I prefer the "dumb" and "safe" way.
But, I could overestimate the problem, because it looks like nobody
cares about it..
The solution introduced for tapes concurrent with iSCSI (which
motivated the need for command-queuing for tapes, since some envisioned
backing up to a tape drive located on 3000 miles away is something
called "unit-attention interlock", or "UA interlock". Check out page
287 of the draft revision 23 of the SCSI Primary Commands - 3 (SPC-3)
standard from T10.org. The UA_INTLCK_CTRL field can be set to cause a
persistent unit attention condition if a command was rejected with
TASK_SET_FULL or BUSY.
Thanks, I'll take a look.
This requires the cooperation of the initiator.
Which practically means that it will not work for at least several
years. I think, I won't be wrong, if say that no Linux initiators use
this feature and going to use...
BTW, it is also impossible to correctly process commands errors (CHECK
CONDITIONs) in async environment without using ACA (Auto Contingent
Allegiance). Again, I see no sign that it's used by Linux or somebody
interested to use it in Linux. Have I missed anything and it is not
important? (rather rhetorical question)
Thanks,
Vlad
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