Mail storage on shared disks
Dovecot also supports keeping mails and index files on NFS. Everything described in this page applies to NFS as well, but see NFS for more NFS-specific problems and optimizations.
By default Dovecot mmap()s the index files. This may not work with all clustered filesystems, and it most certainly won't work with NFS. Setting mmap_disable = yes disables mmap() and Dovecot does its own internal caching. If mmap() is supported by your filesystem, it's still not certain that it gives better performance. Try benchmarking to make sure.
Dovecot supports locking index files with fcntl (default), flock or dotlocks. Some clustered filesystems may not support fcntl, so you can change it to use flock instead. Fcntl locks may also cause problems with some NFS configurations, in which case you can try if switching to dotlocks helps. Note that dotlocks are the slowest locking method. You can change the locking method from lock_method setting.
Regardless of the lock_method setting, Dovecot always uses dotlocks for some locks. Because of this, setting dotlock_use_excl=yes will also improve the performance (default in v1.1+). Before doing that with NFS, make sure that your NFS client and server support O_EXCL (NFSv3+ should be fine).
Run ntpd in each computer to make sure their clocks are synchronized. If the clocks are more than one second apart from each others and multiple computers access the same mailbox simultaneously, you may get errors.
Your cluster will probably perform better if users are usually redirected to the same server. This is because the mailbox may already be cached in the memory and it may also reduce the traffic between the clusterfs nodes. At the very least make sure that the concurrent connections coming for the user from the same IP address go to the same server.
FUSE / GlusterFS
FUSE caches dentries and file attributes internally. If you're using multiple GlusterFS clients to access the same mailboxes, you're going to have problems. With v1.1 these can be avoided by using NFS cache flushes, which just happen to work with FUSE as well:
mail_nfs_index = yes mail_nfs_storage = yes