I/O to pool appears to be hung, panic !

Mon Jul 3 15:27:25 UTC 2017

2017-07-03 14:36 GMT+01:00 Ben RUBSON <ben.rubson at gmail.com>:

> > On 03 Jul 2017, at 13:10, Edward Napierala <trasz at freebsd.org> wrote:
> >
> > 2017-07-03 10:07 GMT+01:00 Ben RUBSON <ben.rubson at gmail.com <mailto:
> ben.rubson at gmail.com>>:
> >
> > > On 29 Jun 2017, at 15:36, Ben RUBSON <ben.rubson at gmail.com <mailto:
> ben.rubson at gmail.com>> wrote:
> > >
> > >> On 29 Jun 2017, at 14:43, Fabian Keil <freebsd-listen at fabiankeil.de
> <mailto:freebsd-listen at fabiankeil.de>> wrote:
> > >
> > > Thank you for your feedback Fabian.
> > >
> > >> Ben RUBSON <ben.rubson at gmail.com <mailto:ben.rubson at 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 at 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.