NFS

NFS is commonly used in one of these ways:

  1. Dovecot is run in a single computer.
  2. Dovecot is run in multiple computers, users are redirected more or less randomly to different computers.
  3. Dovecot is run in multiple computers, each user is assigned a specific computer which is used whenever possible.

You should try to avoid the 2nd setup, see below.

Dovecot configuration

For a fully NFS-safe multi-server setup: 

mmap_disable = yes
dotlock_use_excl = no # only needed with NFSv2, NFSv3+ supports O_EXCL and it's faster
mail_nfs_storage = yes # v1.1+ only
mail_nfs_index = yes # v1.1+ only

Common issues

Clock synchronization

Run ntpd in the NFS server and all the NFS clients 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.

NFS caching problems

NFS caching is a big problem when multiple computers are accessing the same mailbox simultaneously. The best fix for this is to prevent it from happening. Configure your setup so that a user always gets redirected to the same server (unless it's down).

Dovecot v1.1 flushes NFS caches when needed if you set mail_nfs_storage=yes (and mail_nfs_index=yes if indexes are on NFS). This makes the performance slightly worse though.

Dovecot v1.0 doesn't support flushing NFS caches. There's no completely safe way to do this with v1.0, but it helps quite a lot if you disable NFS attribute cache. This can usually be done by giving actimeo=0 or noac mount option. Note that doing this makes the performance much worse, could be as bad as 5 times slower for some operations.

Index files

As described above, it's better to redirect users to the same server whenever possible. If you do this, then it's also a good idea to keep index files stored locally in that server. If the user gets occasionally redirected to another server, the indexes will then be created locally there. This isn't a problem. However you might want to create a cronjob to delete old index directories.

If you choose to keep the index files stored in NFS, you'll need to set mmap_disable=yes. This doesn't affect the performance much, but it makes Dovecot use somewhat more memory.

If you're not running lockd you'll have to set lock_method=dotlock. This makes the performance worse though. Note that many NFS installations have problems with lockd. If you're beginning to get all kinds of locking related errors, try if the problems go away with dotlocking.

Indexes and POP3

The below issue is fixed since v1.1:

If you keep index files in local disks and you move a large number of POP3 users with a lot of mails to a different server, you'll most likely see a huge increase in disk I/O. This is because Dovecot needs to get the "virtual size" of all the messages, which requires reading all the messages' contents. The sizes are then stored into dovecot.index.cache file so the reading isn't needed to be done the next time.

To avoid this disk I/O increase, you can copy the users' dovecot.index and dovecot.index.cache files to the new server before the actual move is done.

With Maildir you can also use the special ,W=<vsize> field in the filename. See MailboxFormat/Maildir.

Single computer setup

This doesn't really differ from keeping mails stored locally. For better performance you should keep index files stored in a local disk.

Random redirects to multiple servers

You should avoid this setup whenever possible. Besides the NFS cache problems described above, mailbox contents can't be cached as well in the memory either. This is more problematic with mbox than with maildir, but in both cases if a client is redirected to a different server when reconnecting, the new server will have to read some data via the NFS into memory, while the original server might have had the data already cached.

If you choose to use this setup, at the very least try to make connections from a single IP redirected into the same server. This avoids the biggest problems with clients that use multiple connections.

Per-user redirects to multiple servers

This method performs a lot better than random redirects. It maximizes the caching possibilities and prevents the problems caused by simultaneous mailbox access.

New mail deliveries are often still handled by different computers. This isn't a problem with maildir as long as you're not using Dovecot LDA (i.e. dovecot-uidlist file or index files shouldn't get updated). It shouldn't be a problem with mboxes either as long as you're using fcntl locking. v1.1 fixes this also with mail_nfs_* settings.

NFS clients

Here's a list of kernels that have been tried as NFS clients:

Misc notes

NFS (last edited 2008-05-17 01:22:14 by TimoSirainen)