[Dovecot] docecot and procmail?

[Thomas Wouters]

On Wed, Apr 27, 2005 at 12:44:47AM +0300, Timo Sirainen wrote:

> The problem [with dovecot and NFS] isn't in either mbox or maildir, it's
> Dovecot's index files. It's of course possible to store indexes to local
> disk, but if you're using multiple servers you'll get worse performance
> whenever latest indexes can't be used.

> Recent 1.0-tests should be somewhat usable with NFS too. I think someone
> in this list already is using it.

We've actually been using Dovecot with mailboxes (both mbox and maildir)
stored on NFS mounts. Only a few people use it regularly, so far, but we
haven't seen any issues lately. I think, anyway :) Index files are stored on
local disks, and yet we're using multiple servers; the layer-4 ethernet
switch (which does the balancing between servers) tries to send new
connections from the same IP address to the same server, in order to re-use
the indices.

Maildir is supposed to be NFS-safe anyway, no locking required; renames (and
other metadata updates) are supposed to be atomic even over NFS, and they
certainly are in the case of our NFS setup (which has grown to 4 NetApp
Filers and ~70 FreeBSD clients -- with sendmail, procmail, qmail-pop3d,
UW-IMAP, uqwk, elm, mutt, pine, dovecot, the whole lot as clients.) We've
been using the same setup to handle quite a lot of email for quite a lot of
years.

'mbox' mailboxes do need to be locked, though; 'dotlocking' is considered
the only reasonable way to lock over NFS, and both Dovecot and Procmail
support this. (You can actually use fcntl/flock over NFS if the server and
client both support lockd, but good lockd implementations are scarce and
even the good ones suffer greatly from the locking deficiency in NFS; NFS
doesn't support locking very well, because it's essentially stateless, and
locks are by definition state. lockd tries to work around this, but it has
complications. Crashed servers leaving unbreakable locks until those servers
return, for instance.)

When compiling procmail, you may want to tell it explicitly to use
dotlocking (by defining all of NO_fcntl_LOCK, NO_lockf_LOCK and
NO_flock_LOCK, in config.h). Procmail's locktest isn't completely reliable
as on some systems, flock/fcntl/lockf may actually seem to lock properly --
locally, even without lockd -- but the lock won't be propagated over NFS and
so it's completely useless. However, if you run procmail's locktest (and
tell it on which NFS filesystems to test) and it tells you:

/* Procmail will lock via: dotlocking */

... it all should work.

-- 
Thomas Wouters <thomas at xs4all.net>

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