> -----Original Message----- > From: ceph-users [mailto:ceph-users-bounces at lists.ceph.com] On Behalf Of > Gregory Farnum > Sent: 20 February 2017 22:13 > To: Nick Fisk <nick at fisk.me.uk>; David Zafman <dzafman at redhat.com> > Cc: ceph-users <ceph-users at ceph.com> > Subject: Re: [ceph-users] How safe is ceph pg repair these days? > > On Sat, Feb 18, 2017 at 12:39 AM, Nick Fisk <nick at fisk.me.uk> wrote: > > From what I understand in Jewel+ Ceph has the concept of an > > authorative shard, so in the case of a 3x replica pools, it will > > notice that 2 replicas match and one doesn't and use one of the good > > replicas. However, in a 2x pool your out of luck. > > > > However, if someone could confirm my suspicions that would be good as > well. > > Hmm, I went digging in and sadly this isn't quite right. The code has a lot of > internal plumbing to allow more smarts than were previously feasible and > the erasure-coded pools make use of them for noticing stuff like local > corruption. Replicated pools make an attempt but it's not as reliable as one > would like and it still doesn't involve any kind of voting mechanism. > A self-inconsistent replicated primary won't get chosen. A primary is self- > inconsistent when its digest doesn't match the data, which happens when: > 1) the object hasn't been written since it was last scrubbed, or > 2) the object was written in full, or > 3) the object has only been appended to since the last time its digest was > recorded, or > 4) something has gone terribly wrong in/under LevelDB and the omap entries > don't match what the digest says should be there. > Thanks for the correction Greg. So I'm guessing that the probability of overwriting with an incorrect primary is reduced in later releases, but it can still happen. Quick question and its maybe that this is a #5 on your list. What about objects that are marked inconsistent on the primary due to a read error. I would say 90% of my inconsistent PG's are always caused by a read error and associated smartctl error. "rados list-inconsistent-obj" shows that it knows that the primary had a read error, so I assume a "pg repair" wouldn't try and read from the primary again? > David knows more and correct if I'm missing something. He's also working on > interfaces for scrub that are more friendly in general and allow > administrators to make more fine-grained decisions about recovery in ways > that cooperate with RADOS. > -Greg > > > > >> -----Original Message----- > >> From: ceph-users [mailto:ceph-users-bounces at lists.ceph.com] On Behalf > >> Of Tracy Reed > >> Sent: 18 February 2017 03:06 > >> To: Shinobu Kinjo <skinjo at redhat.com> > >> Cc: ceph-users <ceph-users at ceph.com> > >> Subject: Re: [ceph-users] How safe is ceph pg repair these days? > >> > >> Well, that's the question...is that safe? Because the link to the > >> mailing > > list > >> post (possibly outdated) says that what you just suggested is > >> definitely > > NOT > >> safe. Is the mailing list post wrong? Has the situation changed? > >> Exactly > > what > >> does ceph repair do now? I suppose I could go dig into the code but > >> I'm > > not > >> an expert and would hate to get it wrong and post possibly bogus info > >> the the list for other newbies to find and worry about and possibly > >> lose their data. > >> > >> On Fri, Feb 17, 2017 at 06:08:39PM PST, Shinobu Kinjo spake thusly: > >> > if ``ceph pg deep-scrub <pg id>`` does not work then > >> > do > >> > ``ceph pg repair <pg id> > >> > > >> > > >> > On Sat, Feb 18, 2017 at 10:02 AM, Tracy Reed > >> > <treed at ultraviolet.org> > >> wrote: > >> > > I have a 3 replica cluster. A couple times I have run into > >> > > inconsistent PGs. I googled it and ceph docs and various blogs > >> > > say run a repair first. But a couple people on IRC and a mailing > >> > > list thread from 2015 say that ceph blindly copies the primary > >> > > over the secondaries and calls it good. > >> > > > >> > > http://lists.ceph.com/pipermail/ceph-users-ceph.com/2015- > >> May/001370. > >> > > html > >> > > > >> > > I sure hope that isn't the case. If so it would seem highly > >> > > irresponsible to implement such a naive command called "repair". > >> > > I have recently learned how to properly analyze the OSD logs and > >> > > manually fix these things but not before having run repair on a > >> > > dozen inconsistent PGs. Now I'm worried about what sort of > >> > > corruption I may have introduced. Repairing things by hand is a > >> > > simple heuristic based on comparing the size or checksum (as > >> > > indicated by the logs) for each of the 3 copies and figuring out > >> > > which is correct. Presumably matching two out of three should win > >> > > and the odd object out should be deleted since having the exact > >> > > same kind of error on two different OSDs is highly improbable. I > >> > > don't understand why ceph repair wouldn't have done this all along. > >> > > > >> > > What is the current best practice in the use of ceph repair? > >> > > > >> > > Thanks! > >> > > > >> > > -- > >> > > Tracy Reed > >> > > > >> > > _______________________________________________ > >> > > ceph-users mailing list > >> > > ceph-users at lists.ceph.com > >> > > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > >> > > > >> > >> -- > >> Tracy Reed > > > > _______________________________________________ > > ceph-users mailing list > > ceph-users at lists.ceph.com > > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > _______________________________________________ > ceph-users mailing list > ceph-users at lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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