[DRBD-user] HA-NFS & drbd

[Eugene Crosser]

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 theory.

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...

Eugene
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