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.txt>.

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> [LDA.txt] (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:

 * FreeBSD has a caching bug
   [http://www.freebsd.org/cgi/query-pr.cgi?pr=123755] which causes problems
   when mailbox is being accessed from different computers at the same time
 * Linux 2.6.16: 'utime()' is buggy, fix in here
   [http://client.linux-nfs.org/Linux-2.6.x/2.6.16/linux-2.6.16-007-fix_setattr_clobber.dif].
   With the fix applied, utime() seems to work perfectly.  High-volume systems
   may experience VFS lock sync issues and for these the complete patchset at
   http://www.linux-nfs.org/Linux-2.6.x/2.6.16/linux-2.6.16-NFS_ALL.dif is
   suggested and appears to work well in production.
 * Linux 2.6.18: Seems to have intermittent caching issues. The same .config
   with 2.6.20.1 has been tested and appears to work well.
 * Linux 2.4.8: Has caching problems, don't know if they can be solved
 * Solaris: If it's completely broken, see
   http://dovecot.org/list/dovecot/2006-December/018145.html
 * The Connectathon test suite is very useful to verify a healthy NFS setup,
   see http://www.connectathon.org/nfstests.html

Misc notes
==========

 * Dovecot doesn't care about root_squash setting, all the root-owned files are
   in /var/run typically which is not in NFS
 * In an environment using Debian (2.6.18) clients with Isilon NFS cluster
   nodes - the following mount options were found to be the most
   successful:'rsize=32768,wsize=32768,hard,fg,lock,nfsvers=3,tcp,retrans=0,actimeo=0
    0 0'
    * As explained above, actimeo=0 will make the performance bad. With v1.1
      use mail_nfs_* settings instead.
 * To learn more about NFS caching and other issues, mostly from a programmer's
   point of view, see NFS Coding HOWTO [http://www.unixcoding.org/NFSCoding] 

(This file was created from the wiki on 2009-01-05 04:42)
