[Dovecot] Replication plans

Christian Balzer chibi at gol.com
Fri May 18 05:41:46 EEST 2007

On Thu, 17 May 2007 19:17:25 +0300 Timo Sirainen <tss at iki.fi> wrote:

> On Thu, 2007-05-17 at 10:04 -0500, Troy Benjegerdes wrote:
> > But that's currently not *really* replicated. The real question I guess
> > is why not use a cluster/distributed/san filesystem like AFS, GFS,
> > Lustre, GPFS to handle the actual data, and specify that replication
> > only works for maildir or other single file per message mailbox
> > formats.
> This already works, but people don't seem to want to use it. There are
> apparently some stability, performance, complexity and whatever
> problems. And if you're planning on replicating to a remote location far
> away they're really slow. But I haven't personally tried any of them, so
> all I can say is that people are interested in getting replication that
> doesn't require clustered filesystems, so I'll build it for them. :)
I for one would rather pay you for not re-inventing the wheel, but
if people with actual access to funds are willing to pay you for this
then I guess "take the money and run" is the thing to do. :-p

Yes, all these FS based approaches currently have one or more of
the issues Timo lists. The question of course is, will a replicated
dovecot be less complex, slow, etc.
For people with money, there are enterprise level replicated file
systems and/or hardware replicated SANs (remote locations, too).
For people w/o those funds there are the above approaches (which
despite all their shortcomings can work, right now) and of course 
one frequently overlooked but perfectly fitting solution, DRBD.
For the ZFS fanbois, there ware ways to make it clustered/replicated
as well (some storageworks add-on or geom tricks).

The point (for me) would be to not just replicate IMAP (never mind
that most of our users use POP, me preferring not to use the 
dovecot LDA, etc), but ALL of the services/infrastructure that make
up a mail system. Which leads quickly back to HA/cluster/SAN/DRBD
for me. 

> > I also tend to have imap clients open on multiple machines, so the
> > assumption that a user's mailbox will only be accessed from 1 IP is
> > probably a bad one.
> Yes, I know. But usually there's only one client that's actively
> modifying the mailbox. The readers don't need global locking because
> they're not changing anything. Except \Recent flag updates..
We have multiple ACTIVE clients accessing the same mailbox all the
time. Company role accounts love that kind of setup. ^_^


Christian Balzer        Network/Systems Engineer                NOC
chibi at gol.com   	Global OnLine Japan/Fusion Network Services

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