Date: Mon, 3 Jul 2017 16:27:23 +0100 From: Edward Napierala <trasz@freebsd.org> To: Ben RUBSON <ben.rubson@gmail.com> Cc: Freebsd fs <freebsd-fs@freebsd.org>, freebsd-scsi <freebsd-scsi@freebsd.org> Subject: Re: I/O to pool appears to be hung, panic ! Message-ID: <CAFLM3-pzOMHmd4PVvZRxe6GnmdpH2-tTAQXjhw8MuU9Y1-oRxQ@mail.gmail.com> In-Reply-To: <1F414ECE-1856-4EA3-A141-88B64703D4D6@gmail.com> References: <E8CC223E-3F41-4036-84A9-FBA693AC2CAA@gmail.com> <20170629144334.1e283570@fabiankeil.de> <A1CC7D73-4196-4503-9716-52E84AA24FD3@gmail.com> <C584B1DF-AC6E-4E77-9497-3D0EED76EACF@gmail.com> <CAFLM3-qpsGx=EYHxAaLsSzF22JAJx0zg8deJ3FX_ec5uDO=0Cw@mail.gmail.com> <1F414ECE-1856-4EA3-A141-88B64703D4D6@gmail.com>
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2017-07-03 14:36 GMT+01:00 Ben RUBSON <ben.rubson@gmail.com>: > > On 03 Jul 2017, at 13:10, Edward Napierala <trasz@freebsd.org> wrote: > > > > 2017-07-03 10:07 GMT+01:00 Ben RUBSON <ben.rubson@gmail.com <mailto: > ben.rubson@gmail.com>>: > > > > > On 29 Jun 2017, at 15:36, Ben RUBSON <ben.rubson@gmail.com <mailto: > ben.rubson@gmail.com>> wrote: > > > > > >> On 29 Jun 2017, at 14:43, Fabian Keil <freebsd-listen@fabiankeil.de > <mailto:freebsd-listen@fabiankeil.de>> wrote: > > > > > > Thank you for your feedback Fabian. > > > > > >> Ben RUBSON <ben.rubson@gmail.com <mailto:ben.rubson@gmail.com>> > wrote: > > >> > > >>> One of my servers did a kernel panic last night, giving the > following message : > > >>> panic: I/O to pool 'home' appears to be hung on vdev guid 122... at > '/dev/label/G23iscsi'. > > >> [...] > > >>> Here are some numbers regarding this disk, taken from the server > hosting the pool : > > >>> (unfortunately not from the iscsi target server) > > >>> https://s23.postimg.org/zd8jy9xaj/busydisk.png < > https://s23.postimg.org/zd8jy9xaj/busydisk.png>; > > >>> > > >>> We clearly see that suddendly, disk became 100% busy, meanwhile CPU > was almost idle. > > > > We also clearly see that 5 minutes later (02:09) disk seems to be back > but became 100% busy again, > > and that 16 minutes later (default vfs.zfs.deadman_synctime_ms), panic > occurred. > > > > >>> No error message at all on both servers. > > >> [...] > > >>> The only log I have is the following stacktrace taken from the > server console : > > >>> panic: I/O to pool 'home' appears to be hung on vdev guid 122... at > '/dev/label/G23iscsi'. > > >>> cpuid = 0 > > >>> KDB: stack backtrace: > > >>> #0 0xffffffff80b240f7 at kdb_backtrace+0x67 > > >>> #1 0xffffffff80ad9462 at vpanic+0x182 > > >>> #2 0xffffffff80ad92d3 at panic+0x43 > > >>> #3 0xffffffff82238fa7 at vdev_deadman+0x127 > > >>> #4 0xffffffff82238ec0 at vdev_deadman+0x40 > > >>> #5 0xffffffff82238ec0 at vdev_deadman+0x40 > > >>> #6 0xffffffff8222d0a6 at spa_deadman+0x86 > > >>> #7 0xffffffff80af32da at softclock_call_cc+0x18a > > >>> #8 0xffffffff80af3854 at softclock+0x94 > > >>> #9 0xffffffff80a9348f at intr_event_execute_handlers+0x20f > > >>> #10 0xffffffff80a936f6 at ithread_loop+0xc6 > > >>> #11 0xffffffff80a900d5 at fork_exit+0x85 > > >>> #12 0xffffffff80f846fe at fork_trampoline+0xe > > >>> Uptime: 92d2h47m6s > > >>> > > >>> I would have been pleased to make a dump available. > > >>> However, despite my (correct ?) configuration, server did not dump : > > >>> (nevertheless, "sysctl debug.kdb.panic=1" make it to dump) > > >>> # grep ^dump /boot/loader.conf /etc/rc.conf > > >>> /boot/loader.conf:dumpdev="/dev/mirror/swap" > > >>> /etc/rc.conf:dumpdev="AUTO" > > >> > > >> You may want to look at the NOTES section in gmirror(8). > > > > > > Yes, I should already be OK (prefer algorithm set). > > > > > >>> I use default kernel, with a rebuilt zfs module : > > >>> # uname -v > > >>> FreeBSD 11.0-RELEASE-p8 #0: Wed Feb 22 06:12:04 UTC 2017 > root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC > > >>> > > >>> I use the following iSCSI configuration, which disconnects the disks > "as soon as" they are unavailable : > > >>> kern.iscsi.ping_timeout=5 > > >>> kern.iscsi.fail_on_disconnection=1 > > >>> kern.iscsi.iscsid_timeout=5 > > >>> > > >>> I then think disk was at least correctly reachable during these 20 > busy minutes. > > >>> > > >>> So, any idea why I could have faced this issue ? > > >> > > >> Is it possible that the system was under memory pressure? > > > > > > No I don't think it was : > > > https://s1.postimg.org/uvsebpyyn/busydisk2.png < > https://s1.postimg.org/uvsebpyyn/busydisk2.png>; > > > More than 2GB of available memory. > > > Swap not used (624kB). > > > ARC behaviour seems correct (anon increases because ZFS can't actually > write I think). > > > Regarding the pool itself, it was receiving data at 6MB/s, sending > around 30kB blocks to disks. > > > When disk went busy, throughput fell to some kB, with 128kB blocks. > > > > > >> geli's use of malloc() is known to cause deadlocks under memory > pressure: > > >> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209759 < > https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209759>; > > >> > > >> Given that gmirror uses malloc() as well it probably has the same > issue. > > > > > > I don't use geli so I should not face this issue. > > > > > >>> I would have thought ZFS would have taken the busy device offline, > instead of raising a panic. > > >>> Perhaps it is already possible to make ZFS behave like this ? > > >> > > >> There's a tunable for this: vfs.zfs.deadman_enabled. > > >> If the panic is just a symptom of the deadlock it's unlikely > > >> to help though. > > > > > > I think this tunable should have prevented the server from having > raised a panic : > > > # sysctl -d vfs.zfs.deadman_enabled > > > vfs.zfs.deadman_enabled: Kernel panic on stalled ZFS I/O > > > # sysctl vfs.zfs.deadman_enabled > > > vfs.zfs.deadman_enabled: 1 > > > > > > But not sure how it would have behaved then... > > > (busy disk miraculously back to normal status, memory pressure due to > anon increasing...) > > > > I then think it would be nice, once vfs.zfs.deadman_synctime_ms has > expired, > > to be able to take the busy device offline instead of raising a panic. > > Currently, disabling deadman will avoid the panic but will let the > device slowing down the pool. > > > > I still did not found the root cause of this issue, not sure I will, > > quite difficult actually with a stacktrace and some performance graphs > only :/ > > > > What exactly is the disk doing when that happens? What does "gstat" > say? If the iSCSI > > target is also FreeBSD, what does ctlstat say? > > As shown on this graph made with gstat numbers from initiator : > https://s23.postimg.org/zd8jy9xaj/busydisk.png > The disk is continuously writing 3 MBps before the issue happens. > When it occurs, response time increases to around 30 seconds (100% busy), > and consequently disk throughput drops down to some kBps. > CPU stays at an almost fully idle level. > > As shown here, no memory pressure : > https://s1.postimg.org/uvsebpyyn/busydisk2.png <https://s1.postimg.org/ > uvsebpyyn/busydisk2.png> > > At the end of graphs' lines, panic is raised. > > iSCSI target is also FreeBSD, unfortunately ctlstat was not running during > the issue occurred. > So numbers will be average since system startup (102 days ago). > I also do not have gstat numbers from this disk on target side > (to help finding if it's a hardware issue, a iSCSI issue or something > else). > I will think about collecting these numbers if ever issue occurs again. > It's kind of hard to say something definitive at this point, but I suspect it's a problem at the target side. I got a report about something quite similar some two years ago, and it turned out to be a problem with a disk controller on the target.
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