dovecot - Jan 2007 - Current state of mulit-server NFS setups

Hi list,

I've been perusing the list archives, but I couldn't find a recent, 
definitive answer to what I'd like to know. I'm looking to set up a new 
batch of servers to migrate ~160,000 accounts, spread across several 
domains, onto (from CommuniGate). Last year I worked with the earlier 
beta/RC versions of Dovecot, so I'm generally familiar with the setup 
required. However, a lot of development has been done since I was last 
dealing with it.

So, what I'd like to end up with is a load balanced group of servers 
providing pop and imap access with quota support using an NFS 
filesystem. We have a new Netapp device (forget the actual model) that 
can do SAN/iSCSI/NFS. We tried using various clustered filesystems 
previously, but the performance/stability issues were a nightmare, so 
I've fallen back to NFS (or perhaps SAN if users can't hit any server in
the group).

So, what does my dovecot setup need to be? I've seen some people say 
they disable indexing in the LDA, but allow pop/imap to index. Some say 
the attribute cache needs to be disabled, which I'd think would be worse 
than just disabling indexes. Some say indexes should be kept on local 
disk (dedicated server for users), and others seem to report problems 
even when doing so. Is there a general consensus?

I was also wondering if dovecot's sanity checking on the indexes would 
allow each server in a balanced group to keep indexes on local disk (so 
if the maildir has changed on another server, dovecot rebuilds it). If 
so, does that take away all performance gains from having indexes at all?

Thanks for any info you all can provide.

On Tue, 2007-01-23 at 09:29 -0500, Justin McAleer wrote:
> 
> 
> So, what does my dovecot setup need to be? I've seen some people say 
> they disable indexing in the LDA, but allow pop/imap to index. Some
> say 
> the attribute cache needs to be disabled, which I'd think would be
> worse 
> than just disabling indexes. Some say indexes should be kept on local 
> disk (dedicated server for users), and others seem to report problems 
> even when doing so. Is there a general consensus?
I don't think there are any problems with keeping indexes in local disk,
other than what generic index file bugs there may be.
> I was also wondering if dovecot's sanity checking on the indexes
> would 
> allow each server in a balanced group to keep indexes on local disk
> (so 
> if the maildir has changed on another server, dovecot rebuilds it). 
Yes, it would.
> If so, does that take away all performance gains from having indexes
> at all?
If the server changes often, probably..

I think your possibilities are (best performance -> lowest performance):

a) Try to redirect the users into the same server always, so that you
can keep indexes in local disks.

b) Try to redirect the users into the same server, but keep index files
in NFS. You don't need to disable attribute cache, as long as you're not
redirecting the same user into two different servers at the same time
(or within NFS attribute cache TTL more exactly).

c) Disable index files and redirect the users wherever.
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