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. You can use <director>
[Director.txt] service to easily implement the 3rd setup.
Dovecot configuration
---------------------
Multi-server setup that tries to flush NFS caches:
---%<-------------------------------------------------------------------------
mmap_disable = yes
dotlock_use_excl = no # only needed with NFSv2, NFSv3+ supports O_EXCL and it's
faster
mail_nfs_storage = yes
mail_nfs_index = yes
---%<-------------------------------------------------------------------------
High performance NFS setup with indexes on local disk (see below for benefits):
---%<-------------------------------------------------------------------------
mmap_disable = no
dotlock_use_excl = yes # You most likely have NFSv3
mail_nfs_storage = yes
mail_nfs_index = no
---%<-------------------------------------------------------------------------
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). This also means that mail deliveries must be done by the
same server, or alternatively it shouldn't update index files.
Dovecot flushes NFS caches when needed if you set 'mail_nfs_storage=yes', but
unfortunately this doesn't work 100%, so you can get random errors.
Disabling NFS attribute cache helps a lot in getting rid of caching related
errors, but this makes the performance MUCH worse and increases the load on NFS
server. This can usually be done by giving 'actimeo=0' or 'noac' mount option.
Index files
-----------
As described above, it's better to redirect users to the same server whenever
possible. If you do this, it might also be a good idea to keep index files
stored locally in that server. If 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'. Both the 'mmap_disable' and indexing to NFS will result in
a notable performance hit. If you're not running lockd you'll have to set
'lock_method=dotlock', but this degrades performance. 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.
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 <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. This problem
can be fully solved by using LMTP protocol to deliver the mails to the correct
server (possibly using Dovecot's LMTP proxy).
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 2011-11-16 14:09)