Restarting exports disturbs clients

[Daniel Feenberg]

When we change the exportfs file on our FreeBSD 9.1 fileserver and signal
mountd to reread the file:

   kill -HUP `cat /var/run/mountd.pid`

it kills the jobs on  clients that have files open on the fileserver. They
terminate with an I/O error. The same thing happens if NFS is restarted.

This is pretty inconvenient for users (and us). Is there a way around this? We 
have noticed that a Linux fileserver can restart nfs without distrubing clients 
(other than a short pause). The Linux restart doesn't restart the locking 
mechanism - is that the difference? We could do without locks, even 
without NFSv4, for that matter, if it would let us change exports without 
disturbing users. Perhaps there there is an NFS shutdown procedure that we 
should be using?

Daniel Feenberg
NBER