[Dovecot] netapp/maildir/dovecot performance

[bofh list]

We are seeing some poor performance recently that is focused around
users with large mailboxes (100,000 message /INBOX, 80,000 message
subfolders, etc).

The performance problem manifests as very high system% utilization -
basically iowait for NFS.

There are two imap servers with plenty of horsepower/memory/etc.  They
are connected to a 3050c cluster via gig-e.  Here are the mount
options: rw,tcp,nfsvers=3,rsize=8192,wsize=8192,nosuid,nodev,soft,intr,noac,actimeo=0

I need to get this system% utilization brought down because it is
starting to slowdown imap performance for other users (it goes up to
95% on a regular basis, with users that have folders with many
messages).

My thought is to try removing the noac option and bump up the actimeo
value from 0 to 30.  Unfortunately I don't have access to use a SLB to
direct traffic to individual servers.

My understanding is that the worst that can happen is index corruption
(which will result in rebuilding them when it happens).

Any thoughts?  I'm running RC24.