[DRBD-user] HA-NFS & drbd
crosser at rol.ru
Wed Mar 2 17:01:58 CET 2005
Brett Bolen wrote:
> I would like to run High-Availability NFS on top of drbd, as in the
> drbd.org articles.
> What will happen when the first device fails, and the secondary
> takes over? Will I lose uncommitted data from the nfs clients?
> Will by apps need to be restarted ( if they have files open).
> What if my home directory is on the NFS filesystem? Will
> I need to logout and login again?
> Does /var/lib/nfs in a shared directory take care of these things?
> I've seen cases where the file system goes away ( without drbd),
> but the nfs clients continue to operated as if there was a
> file system ( nfs caching?).
Unlike SMB, NFS server failover, when done carefully, is completely
transparent to the clients, that is, when a client was in the middle of
writing a file at the moment of failover, the operation will continue
after a pause and in the end you will get complete correct file.
In practice there are some caveats.
First, if your clients rely on fcntl locking, you may be in trouble. I
mount NFS shares with -o nolock, and consequently locking just does not
work (which is OK for my applications but may be bad for yours).
Second, *if* you export shares for networks rathar than individual IP
addresses, you *must* have /var/lib/nfs symlinked (or mounted) onto
filesystem residing on DRBD device. Otherwise you will get EPERM errors
after failover, and will have to remount shares on the clients.
Third, I have (rarely) observed misterious behavior such as "Stale NFS
handle" errors after failover. This should not happen if you have tidy
setup with matching versions of everything, though...
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