Busy NFS device - what's causing it?

[Mark Knecht]

Hi,
   This is jsut a generic question since the list has been pretty
quiet. At time I'll want to unmount a volume. For instance, this
morning I'm messing around a bit with mount options and I want to see
what happens when I try a new option from the command line. However I
cannot unmount the volume to try mounting it:

dragonfly ~ # umount /video
umount: /video: device is busy
umount: /video: device is busy
dragonfly ~ #

   Generically, how do I determine why a device is 'busy'? If the
system thinks it's busy I assume it keeps that state somewhere. Is it
available to me to read? When I grep mtab I see this:

dragonfly ~ # cat /etc/mtab | grep video
myth14:/video /video nfs
rw,noexec,nosuid,nodev,_netdev,addr=192.168.1.14,addr=192.168.1.14 0 0
dragonfly ~ #

...telling me where it's located but it doesn't tell me who's using
it. I happen to know that the only valid user of this partition is
mythbackend which I've shut down, but I still cannot unmount it.

   So, how do I find what's keeping the partition busy?

Cheers,
Mark