Re: safety of retrying SYNCHRONIZE CACHE [was: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush]

[Hannes Reinecke <hare@suse.de>]

Hannes Reinecke wrote:
> Mike Snitzer wrote:
>> Hi Kiyoshi,
>>
>> On Mon, Aug 30 2010 at  2:13am -0400,
>> Kiyoshi Ueda <k-ueda@ct.jp.nec.com> wrote:
>>
>>>> That does seem like a valid concern.  But I'm not seeing why its unique
>>>> to SYNCHRONIZE CACHE.  Any IO that fails on the target side should be
>>>> passed up once the error gets to DM.
>>> See the Tejun's explanation again:
>>>     http://marc.info/?l=linux-kernel&m=128267361813859&w=2
>>> What I'm concerning is whether the same thing as Tejun explained
>>> for ATA can happen on other types of devices.
>>>
>>>
>>> Normal write command has data and no data loss happens on error.
>>> So it can be retried cleanly, and if the result of the retry is
>>> success, it's really success, no implicit data loss.
>>>
>>> Normal read command has a sector to read.  If the sector is broken,
>>> all retries will fail and the error will be reported upwards.
>>> So it can be retried cleanly as well.
>> I reached out to Fred Knight on this, to get a more insight from a pure
>> SCSI SBC perspective, and he shared the following:
>>
>> ----- Forwarded message from "Knight, Frederick" <Frederick.Knight@netapp.com> -----
>>
>>> Date: Tue, 31 Aug 2010 13:24:15 -0400
>>> From: "Knight, Frederick" <Frederick.Knight@netapp.com>
>>> To: Mike Snitzer <snitzer@redhat.com>
>>> Subject: RE: safety of retrying SYNCHRONIZE CACHE?
>>>
>>> There are requirements in SBC to maintain data integrity.  If you WRITE
>>> a block and READ that block, you must get the data you sent in the
>>> WRITE.  This will be synchronized around the completion of the WRITE.
>>> Before the WRITE completes, who knows what a READ will return.  Maybe
>>> all the old data, maybe all the new data, maybe some mix of old and new
>>> data.  Once the WRITE ends successful, all READs of those LBAs (from any
>>> port) will always get the same data.
>>>
>>> As for errors, SBC describes how the deferred errors are reported (like
>>> when a CACHE tries to flush but fails).  So if a write from cache to
>>> media does have problems, the device would tell you via a CHECK
>>> CONDITION (with the first byte of the sense data set to 71h or 73h.  SBC
>>> clause 4.12 and 4.13 cover a lot of this information.  It is these error
>>> codes that prevent silent loss of data.  And, in this case, when the
>>> CHECK CONDITION is delivered, it will have nothing to do with the
>>> command that was issued (the victim command).  If you look into the
>>> sense data, you will see the deferred error flag, and all the additional
>>> information fields will relate to the original I/O
>>>
>>> SYNCHRONIZE CACHE is not substantially different than a WRITE (it puts
>>> data on the media).  So issuing it multiple times wouldn't be any
>>> different than issuing multiple WRITES (it might put a temporary dent in
>>> performance as everything flushes out to media).  If it or any other
>>> commands fail with 71h/73h, then you have to dig down into the sense
>>> data buffer to find out what happened.  For example, if you issue a
>>> WRITE command, and it completes into write back cache but later (before
>>> being written to the media), some of the cache breaks and looses data,
>>> then the device must signal a deferred error to tell the host, and cause
>>> a forced error on the LBA in question.
>>>
>>> Does that help?
>>>
>>>       Fred
>> ----- End forwarded message -----
>>
>> Seems like verifying/improving the handling of CHECK CONDITION is a more
>> pressing concern than silent data loss purely due to SYNCHRONIZE CACHE
>> retries.  Without proper handling we could completely miss these
>> deferred errors.
>>
> Yes.
> 
>> But how to effectively report such errors to upper layers is unclear to
>> me given that a particular SCSI command can carry error information for
>> IO that was already acknowledged successful (e.g. to the FS).
>>
>> drivers/scsi/scsi_error.c's various calls to scsi_check_sense()
>> illustrate Linux's current CHECK CONDITION handling.  I need to look
>> closer at how deferred errors propagate to upper layers.  After an
>> initial look it seems scsi_error.c does handle retrying commands where
>> appropriate.
>>
>> I believe Hannes has concerns/insight here.
>>
> 
> Quite. We _should_ be handling deferred errors correctly;
> if you check drivers/scsi/scsi_lib.c:scsi_io_completion()
> you'll find this:
> 
> 	if (host_byte(result) == DID_RESET) {
> 		/* Third party bus reset or reset for error recovery
> 		 * reasons.  Just retry the command and see what
> 		 * happens.
> 		 */
> 		action = ACTION_RETRY;
> 	} else if (sense_valid && !sense_deferred) {
>                 ...
> 	} else {
> 		description = "Unhandled error code";
> 		action = ACTION_FAIL;
> 	}
> 
> ie for deferred errors we're already aborting the command. Not sure
> if I agree with this bit in drivers/scsi/scsi_lib.c:
> 
> static int scsi_check_sense(struct scsi_cmnd *scmd)
> {
> 	struct scsi_device *sdev = scmd->device;
> 	struct scsi_sense_hdr sshdr;
> 
> 	if (! scsi_command_normalize_sense(scmd, &sshdr))
> 		return FAILED;	/* no valid sense data */
> 
> 	if (scsi_sense_is_deferred(&sshdr))
> 		return NEEDS_RETRY;
> 
> I doubt we can resolve the situation by retrying the command, which
> will be the wrong command to retry anyway. I would rather
> have those retry 'SUCCESS' and add another case in scsi_io_completion()
> to notify us about the deferred error.
> 
Ah. No. That is actually correct. SPC-3 states:
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.

So we're good after all and I would just add this patch:

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index fb841e3..efb4609 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -912,7 +912,10 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int
 good_bytes)
                        break;
                }
        } else {
-               description = "Unhandled error code";
+               if (sense_deferred)
+                       description = "Deferred error";
+               else
+                       description = "Unhandled error code";
                action = ACTION_FAIL;
        }
 
to make the whole situation more transparent.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Markus Rex, HRB 16746 (AG Nürnberg)
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