** 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
+ 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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