== What can keep a ZFS pool busy and prevent it from exporting

Suppose that you have some [[ZFS-based NFS fileservers
ZFSFileserverSetup]], perhaps in some sort of failover configuration,
and that you want to export some pools from one. So you do:

>  # zpool export tank
>  cannot unmount '/x/y': Device busy

The usual set of tools don't show any local processes using the
filesystem (not that there really are any, the server being purely an
NFS server), and there's no actual NFS activity. In short: rather
puzzling. Also annoying, since this stops the pool from exporting.

(This can also happen with '_zfs upgrade -a_', which apparently
unmounts and remounts the filesystems as it upgrades them.)

When this has happened to us, the thing holding the filesystem busy
turned out to be the Solaris NFS lock manager. Temporarily disabling it
in svcadm allowed the pool to export without problems:

>  # svcadm disable svc:/network/nfs/nlockmgr
>  # zpool export tank
>  # svcadm enable svc:/network/nfs/nlockmgr

I suspect that this will lose any locks attached to the filesystem, and
of course it has side effects if you're still allowing NFS traffic to
filesystems in other pools (we weren't).

(My theory is that the NFS lock manager holds filesystems busy when it
has active locks against something on the filesystem. Unfortunately I
don't know of any way of inspecting its state, although I'm sure there
is one. (And using the [[kernel debugger ForcingS10X86CrashDump]] seems
a bit overkill.))

Having written all of this, I checked the fine manual and I see that
'_zpool export_' has a _-f_ option, and I don't think we tried that when
we ran into this problem. However, I feel better knowing what the actual
cause is (and '_zfs upgrade_' has no _-f_, so we'd have had to deal with
the issue sooner or later).