[Bug 237636] bhyve guest ZFS filesystem freezes in zcw->zcw_cv state

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237636

--- Comment #3 from Morgan Davis <freebsd-bugs at morgandavis.net> ---
The aftereffects of the issue affecting my development bhyve system sound very
similar to those in bug #236220, and maybe they are related.

However, this issue doesn't affect 9 other FreeBSD servers w/ ZFS that I run
with a similarly outfitted FreeBSD 12.0-RELEASE-p3, and many are far more
heavily burdened with work in smaller RAM space. The only key difference is
that this deadlock occurs only my very lightly loaded bhyve guest server.

By comparison, my other systems are far more abused. One is a 2GB instance at
DigitalOcean that acts as a data backup repository. It runs MySQL as a
replication server and also the endpoint for lots of rsync pushes. It's
constantly churning disk I/O with 13% swap in use (due to MySQL). In addition,
it has a cron job that regularly cycles ZFS snapshots across 33 datasets for
historical archiving. Never once has it, or any others I manage, froze up like
my bhyve guest. The only time I need to reboot it is for FreeBSD revision
updates or reconfigurations that I want to test to make sure a restart occurs
properly. Right now, it has 28 days uptime.

Given this success everywhere else except for this one bhyve guest instance,
I'm not sure my issue is related to the same one affecting those in bug #236220
which 

I will try to issue the procstat command the next time this happens, but I have
found that trying to run any command from an open bash prompt usually just
stalls as well. I'll omit the pipe to grep to eliminate another possible
deadlock trigger.

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