Hannes Reinecke wrote: > 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. > 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@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Markus Rex, HRB 16746 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
