From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hannes Reinecke Subject: Re: safety of retrying SYNCHRONIZE CACHE [was: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush] Date: Wed, 01 Sep 2010 09:38:24 +0200 Message-ID: <4C7E02F0.2020701@suse.de> References: <4C6E3C1A.50205@ct.jp.nec.com> <4C72660A.7070009@kernel.org> <20100823141733.GA21158@redhat.com> <4C739DE9.5070803@ct.jp.nec.com> <4C73FA8F.5080800@kernel.org> <4C74CD95.1000208@ct.jp.nec.com> <20100825152831.GA8509@redhat.com> <4C7789BE.1060609@ct.jp.nec.com> <20100827134940.GA22504@redhat.com> <4C7B4C23.8020100@ct.jp.nec.com> <20100901005537.GA21466@redhat.com> <4C7E0184.9030502@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <4C7E0184.9030502@suse.de> Sender: linux-fsdevel-owner@vger.kernel.org To: Mike Snitzer Cc: Kiyoshi Ueda , Tejun Heo , tytso@mit.edu, linux-scsi@vger.kernel.org, jaxboe@fusionio.com, jack@suse.cz, linux-kernel@vger.kernel.org, swhiteho@redhat.com, linux-raid@vger.kernel.org, linux-ide@vger.kernel.org, James.Bottomley@suse.de, konishi.ryusuke@lab.ntt.co.jp, linux-fsdevel@vger.kernel.org, vst@vlnb.net, rwheeler@redhat.com, Christoph Hellwig , chris.mason@oracle.com, dm-devel@redhat.com, Frederick.Knight@netapp.com List-Id: dm-devel.ids Hannes Reinecke wrote: > Mike Snitzer wrote: >> Hi Kiyoshi, >> >> On Mon, Aug 30 2010 at 2:13am -0400, >> Kiyoshi Ueda wrote: >> >>>> That does seem like a valid concern. But I'm not seeing why its u= nique >>>> 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=3Dlinux-kernel&m=3D128267361813859&w=3D2 >>> 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 p= ure >> SCSI SBC perspective, and he shared the following: >> >> ----- Forwarded message from "Knight, Frederick" ----- >> >>> Date: Tue, 31 Aug 2010 13:24:15 -0400 >>> From: "Knight, Frederick" >>> To: Mike Snitzer >>> Subject: RE: safety of retrying SYNCHRONIZE CACHE? >>> >>> There are requirements in SBC to maintain data integrity. If you W= RITE >>> 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 WRIT= E. >>> Before the WRITE completes, who knows what a READ will return. May= be >>> 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 (fro= m 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 t= o >>> 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 th= e >>> 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 addit= ional >>> information fields will relate to the original I/O >>> >>> SYNCHRONIZE CACHE is not substantially different than a WRITE (it p= uts >>> data on the media). So issuing it multiple times wouldn't be any >>> different than issuing multiple WRITES (it might put a temporary de= nt in >>> performance as everything flushes out to media). If it or any othe= r >>> commands fail with 71h/73h, then you have to dig down into the sens= e >>> data buffer to find out what happened. For example, if you issue a >>> WRITE command, and it completes into write back cache but later (be= fore >>> being written to the media), some of the cache breaks and looses da= ta, >>> 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 CAC= HE >> retries. Without proper handling we could completely miss these >> deferred errors. >> > Yes. >=20 >> 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 whe= re >> appropriate. >> >> I believe Hannes has concerns/insight here. >> >=20 > Quite. We _should_ be handling deferred errors correctly; > if you check drivers/scsi/scsi_lib.c:scsi_io_completion() > you'll find this: >=20 > if (host_byte(result) =3D=3D DID_RESET) { > /* Third party bus reset or reset for error recovery > * reasons. Just retry the command and see what > * happens. > */ > action =3D ACTION_RETRY; > } else if (sense_valid && !sense_deferred) { > ... > } else { > description =3D "Unhandled error code"; > action =3D ACTION_FAIL; > } >=20 > ie for deferred errors we're already aborting the command. Not sure > if I agree with this bit in drivers/scsi/scsi_lib.c: >=20 > static int scsi_check_sense(struct scsi_cmnd *scmd) > { > struct scsi_device *sdev =3D scmd->device; > struct scsi_sense_hdr sshdr; >=20 > if (! scsi_command_normalize_sense(scmd, &sshdr)) > return FAILED; /* no valid sense data */ >=20 > if (scsi_sense_is_deferred(&sshdr)) > return NEEDS_RETRY; >=20 > 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. >=20 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, uns= igned int good_bytes) break; } } else { - description =3D "Unhandled error code"; + if (sense_deferred) + description =3D "Deferred error"; + else + description =3D "Unhandled error code"; action =3D ACTION_FAIL; } =20 to make the whole situation more transparent. Cheers, Hannes --=20 Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 N=FCrnberg GF: Markus Rex, HRB 16746 (AG N=FCrnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html