[Bug 273772] zpool nested in ZVOL becomes inaccessible when zfsd(8) is running on the host

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273772

--- Comment #4 from Marek Zarychta <zarychtam@plan-b.pwste.edu.pl> ---
(In reply to Alan Somers from comment #3)
>1) What does "zpool status" show on the host, when the guest is unable to boot?

It shows the same as shows when the guest is able to boot. Guest's zpool is
never imported, neither vfs.zfs.vol.recursive is enabled. 

>2) What does "top" show on the host at that time?  Is zfsd spinning the CPU?

Nothing special, among others:

7280 root         18  32    0    12G    32M kqread  10   3:51 100.50% bhyve
6833 root          1  20    0    20M    11M select  26   0:00   0.00% zfsd

It is also probably worth mentioning, that I have had from time to time (very
rarely though) problems with importing that pool (booting bhyve guest VM) in
the past. It begun in stable/13 somewhere between 13.1 and 13.2 release IIRC.
To fix that I had to enable  vfs.zfs.vol.recursive, import zpool from from the
host, perform some writes, for example clear a few snapshots, export zpool and
then the VM booted fine. It wasn't reproducible though. It's worth to mention
that all that time zfsd(8) was running.

Now, after upgrading the bhyve host to stable/14, the problem with booting
bhyve guest VM is 100% reproducible and fully dependent on the state of
zfsd(8).

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