Fix sent to kernel team mailing list for review:
https://lists.ubuntu.com/archives/kernel-team/2019-May/100990.html
** Description changed:
SRU Bionic
== SRU Justification ==
Intensive I/O such as performed by ZFS send/receive can cause hangs because
of monopolization of the global system_taskq. The outcome is that z_zvol hung
tasks occur and I/O gets blocked.
== Fix ==
Upstream ZFS commit 77d8a0f1a4d0b2f59cee63088f7987cb38e66538 ("Fix hung
- z_zvol tasks during 'zfs receive'" fixes this issue, it adds a dedicated
- per-pool prefetch taskq that prevents the traverse code from
+ z_zvol tasks during 'zfs receive'") fixes this issue, it adds a
+ dedicated per-pool prefetch taskq that prevents the traverse code from
monopolizing the global (and limited) system_taskq by inappropriately
scheduling long running tasks on it. This fixes the z_zvol hung tasks. A
trivial backport is required for Bionic ZFS.
== Testcase ==
Perform large send/receives. Occasionally they lock up. With the fix,
this issue is addressed and no more lockups occur. Also must pass the
full ZFS ubuntu autotest tests to prove no regressions occur.
== Regression Potential ==
This fix adds more per-pool prefetch taskq's so we have more kernel
resources being used. There is therefore a very small risk that this may
impact ZFS running on memory and CPU constrained systems. However, the
fix is small, has been upstream for a while and is in Cosmic+ releases
and has not caused any regressions, so I think this is a relatively safe
fix.
------
I have experienced the problems fixed by this commit
https://github.com/zfsonlinux/zfs/pull/7343 a few times on my NAS. The
system hangs completely when it occurs. It looks like 0.7.9 brings other
interesting bug fixes that potentially freeze the system.
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https://bugs.launchpad.net/bugs/1772412
Title:
zfs 0.7.9 fixes a bug (https://github.com/zfsonlinux/zfs/pull/7343)
that hangs the system completely
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