Date: Wed, 22 Nov 2017 17:22:54 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: Youzhong Yang <youzhong@gmail.com> Cc: Shiva Bhanujan <Shiva.Bhanujan@quorum.com>, "cem@freebsd.org" <cem@freebsd.org>, "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org> Subject: Re: zio_done panic in 10.3 Message-ID: <78d712d9-dda3-0411-262e-bb64f9ab46eb@FreeBSD.org> In-Reply-To: <CADpNCvbpSTFjSHHVGV_=-LK27XLSzfZi_gzSa0v-Z=h_msOQuw@mail.gmail.com> References: <3A5A10BE32AC9E45B4A22F89FC90EC0701C367D3D1@QLEXC01.Quorum.local> <5021a016-9193-b626-78cf-54ffa3929e22@FreeBSD.org> <3A5A10BE32AC9E45B4A22F89FC90EC0701C367D562@QLEXC01.Quorum.local> <CAG6CVpVT=4iid4xi0yw3AJe4kbBNEGj6zVCKfozq_-8CgGYfag@mail.gmail.com> <3A5A10BE32AC9E45B4A22F89FC90EC0701C367D636@QLEXC01.Quorum.local> <41e2465d-e1b5-33ce-57b5-49bea6087d9a@FreeBSD.org> <CADpNCvbpSTFjSHHVGV_=-LK27XLSzfZi_gzSa0v-Z=h_msOQuw@mail.gmail.com>
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On 22/11/2017 16:40, Youzhong Yang wrote: > Hi Andriy, > > This is nice! I am 100% sure it's exactly the same issue I experienced and then > reported to illumos mailing list. In all the crash dumps zio->io_done = > l2arc_read_done, so I thought the crash must be related to L2ARC. Once I set > secondarycache=metadata, the frequency of crash went from one per 2 days down to > one per week. I've been puzzled by what could have caused a zio being destroyed > while there's still child zio. Your explanation definitely makes sense! Oh, I now recall seeing your report: https://illumos.topicbox.com/groups/zfs/Tccd8b4463865899e I remember that it raised my interest, but then I forgot about it and didn't correlate it with the latest reports. > By the way, is there a FreeBSD bug report or an illumos bug number tracking this > issue? I would be more than happy to create one if needed, and also test your > potential fix here in our environment. I am not aware of any existing bug report. It would be great if you could open one [ or two :-) ] If you open an illumos issue, please also add George Wilson as a watcher. I think that George is also interested in fixing this issue and he knows the relevant code better than me. Thank you! > On Tue, Nov 21, 2017 at 3:46 PM, Andriy Gapon <avg@freebsd.org > <mailto:avg@freebsd.org>> wrote: > > > > > On 21/11/2017 21:30, Shiva Bhanujan wrote: > > it did get compressed to 0.5G - still too big to send via email. I did send some more debug information by running kgdb on the core file to Andriy, and I'm waiting for any analysis that he might provide. > > Yes, kgdb-over-email turned out to be a far more efficient compression :-) > I already have an analysis based on the information provided by Shiva and by > another user who has the same problem and contacted me privately. > I am discussing possible ways to fix the problem with George Wilson who was very > kind to double-check the analysis, complete it and suggest possible fixes. > > A short version is that dbuf_prefetch and dbuf_prefetch_indirect_done functions > chain new zio-s under the same parent zio (a completion of one child zio may > create another child zio). They do it using arc_read which can create either a > logical zio in most cases or a vdev zio for a read from a cache device (2arc). > zio_done() has a check for the completion of a parent zio's children but that > check is not completely safe and can be broken by the pattern that dbuf_prefetch > can create. So, under some specific circumstances the parent zio may complete > and get destroyed while there is a child zio. > > I believe this problem to be rather rare, but there could be configurations and > workloads where it's triggered more often. > The problem does not happen if there are no cache devices. > > > From: Conrad Meyer [cem@freebsd.org <mailto:cem@freebsd.org>] > > > > Sent: Tuesday, November 21, 2017 9:04 AM > > > > To: Shiva Bhanujan > > > > Cc: Andriy Gapon; freebsd-fs@freebsd.org <mailto:freebsd-fs@freebsd.org> > > > > Subject: Re: zio_done panic in 10.3 > > > > > > > > > > > > > > > > Have you tried compressing it with e.g. xz or zstd? > > > > -- > Andriy Gapon > _______________________________________________ > freebsd-fs@freebsd.org <mailto:freebsd-fs@freebsd.org> mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > <https://lists.freebsd.org/mailman/listinfo/freebsd-fs>; > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org > <mailto:freebsd-fs-unsubscribe@freebsd.org>" > > -- Andriy Gapon
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