Mike Snitzer wrote: > Hi Kiyoshi, > > On Mon, Aug 30 2010 at 2:13am -0400, > Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> 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@xxxxxxxxxx> ----- > >> Date: Tue, 31 Aug 2010 13:24:15 -0400 >> From: "Knight, Frederick" <Frederick.Knight@xxxxxxxxxx> >> To: Mike Snitzer <snitzer@xxxxxxxxxx> >> 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. I'll be sending a patch. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Markus Rex, HRB 16746 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
