

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 # default
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. 


Attribute cache problems
========================


NFS attribute 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), and you can safely keep the attribute cache enabled. 
Dovecot v1.1 flushes attribute cache 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 attribute cache. If you really want to allow multiple computers to access the same mailbox at the same time, you'll need to disable the 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 attribute 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 for 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. It's also pretty safe to keep attribute cache enabled (even 'actimeo=2' helps a lot), because other computers are never accessing the mailbox files. 
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 (ie. dovecot-uidlist file or index files shouldn't get updated). It shouldn't be a problem with mboxes either because locking flushes the attribute cache (at least fcntl locking, do dotlocks?) 


NFS clients
===========


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

 * Linux 2.6.16: 'utime()' is buggy, fix in here [None]. 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. 

(This file was created from the wiki on 2007-12-11 04:42)
