Re: [linux-iscsi-devel] [question] deferred sense

Douglas Gilbert
2005-01-07T00:04:33Z

Tom Coughlan wrote:
> On Wed, 2005-01-05 at 11:44, James Bottomley wrote:
>
>>On Wed, 2005-01-05 at 16:23 +0100, Christoph Hellwig wrote:
>>
>>>On Wed, Jan 05, 2005 at 09:21:12AM -0600, Scott M. Ferris wrote:
>>>
>>>>To be more specific, there were some devices that would fail a command
>>>>and return deferred sense. The command didn't complete at the target,
>>>>and the kernel wasn't retrying it because the sense was deferred
>>>>rather than current. For those devices, the translation produced the
>>>>desired retry.
>>>
>>>Do you remember these devices? Might be worth adding a midlayer
>>>blacklist entry for them.
>>
>>That's certainly possible ... although we'd need a lot more details.
>>Any device that returns deferred sense for a current error is pretty
>>badly broken according to the spec.
>
>
> If a current command returns deferred sense, the SCSI spec. requires
> that the current command shall not have been executed [1]. So, if at
> some point in the past the kernel did not retry a current command that
> returned deferred sense, the iscsi folks would have forced the retry by
> converting deferred sense to current sense. The scenario does not
> require a device that is working incorrectly.
>
> The big flaw in what iscsi did is the case where the deferred sense
> indicates a non-fatal error. In that case, iscsi converts it to current,
> the mitlayer examines it and determines that it does not require a
> retry. This causes the current command to complete to the application
> even though it was not executed by the device.
>
> It looks to me as though the 2.4 iscsi driver is susceptible to this. It
> is probably not seen in practice because disk devices that return
> non-fatal deferred sense are rare (it probably requires the PER bit set
> in the error recovery mode page?). Anyone know for sure?

Tom,
From my reading of SBC-2 (rev 16, 13 Nov 2004) deferred errors
cannot be turned off by a mode page. The VERIFY and
WRITE AND VERIFY commands can be used to make sure blocks get
to the media (or not) without deferred errors. There are also
the "Force Unit Access" (FUA and FUA_NV) bits in the READ and
WRITE commands (but not the 6 byte variants).

The t10 folks have added the idea of non-volatile cache in
a disk (more likely a RAID) which further complicates things.
Now a deferred error could theoretically span a power cycle!

The PER bit in the Read-Write Error Recovery mode page (SBC-2
rev 16 section 6.3.4) controls whether RECOVERED ERRORs are
reported or not. Also if the ARRE bit (for reads) and/or the
AWRE bit (for writes) are set in the same mode
page, the offending block will be remapped (whether a recovered
error is reported on not). If PER is 0 then RECOVERED ERRORs
are not reported. [In any case the Error Counter log pages
should reflect the problems (and perhaps the "grown" defect
list) so smartmontools may be of use.]

> [1] If the task terminates with CHECK CONDITION status and the sense
> data describes a deferred error the command for the terminated task
> shall not have been processed. (SPC-3)

I fear my tinkering with sense descriptor data in
the mid level may have tripped up on this case (if
it wasn't already broken), that is: a deferred error
report must cause the current command to be retried,
even if the deferred error was reporting a
recovered error. This case can only arise when PER=1.

Doug Gilbert