Hi Timo,
We've been running Dovecot with Maildir on NFS for quite a while - since back in the 1.0 days I believe. I'm somewhat new here. Anyway...
The Wiki article on NFS states that 1.1 and newer will flush attribute caches if necessary with mail_nfs_storage=yes. We're running 1.2.8 with that set, as well as mail_nfs_index=yes, mmap_disable=yes and fsync_disable=no. We have a pool of POP/IMAP and SMTP machines that are accessing the maildirs, and can't guarantee any sort of user session affinity to a particular host.
We also mount our NFS shares with 'noac', which is what I'm writing to ask about. I'd like to stop doing that for performance reasons. Do you see any issues with taking that out of the mount options, given our environment?
Thanks,
-Brad