[Bug 231117] I/O lockups inside bhyve vms

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231117

--- Comment #17 from Kristian K. Christensen <kkc at duckpond.dk> ---
So, after some extensive testing over the holidays:

FreeBSD 11.1 with as low as 100mb ARC max, doesn't excert this behavior.

FreeBSD 11.2 and 12.0 with 1024 ARC max and lower, excerts this behavior.

FreeBSD 11.2 and 12.0 with over 4GB ARC max does not hang. (I need to see how
low I can make it go, but so far I've been unable to cause 4GB ARC max to hang)

It may seem odd to want to limit ARC, but we have experienced high ARC usage
and applications (bhyve VMs and MySQL daemons notably) being killed due to OOM,
if we don't enforce ARC limits. This causes a lot of nightly on-call activity
and data loss, which is not really something we care for. Furthermore, this is
on VMs that use a zvol as primary storage, on a host that has a fairly high ARC
limit, so ideally the host handles most of the ARC.

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