[Dovecot] Newbie questions: Load-balanced Dovecot with NFS storage

[Mike Brudenell]

Hi,

I experimented with Dovecot a while back and our site is now taking  
the plunge to switch to it from the UW IMAP server when we rejig the  
topology of our mail store and IMAP servers.

Currently we have a farm of 5 IMAP servers running the UW server.   
Each has locally attached disk used to store people's mailboxes.   
Each user is allocated to one of these five servers and can only  
access their mail via that server.  Our aim is to remove this  
restriction to provide resilience in the event of a server failing.

Our thoughts...

*  We are envisaging having a farm of N (to be determined) servers  
running
    Dovecot.  These will be closed systems: users do not have shell- 
access,
    and can only reach their mail via IMAP.

*  A single mailstore common to all N servers will be hosted on our  
NetApp
    filer and NFS-mounted on each server.  Each server would be running
    Solaris 10.  If we use all-new kit it will be x86-based, or we  
may have
    to redeploy some of our existing SPARC boxes to the new service.

*  We will be converting user mailboxes to be in Maildir format.

*  Our hardware load balancer, which supports persistent sessions,  
will be
    used to distribute users between the servers.

    This could either direct a user to any one of the N servers, or  
could be
    set to 'prefer' a given server for a particular user (only  
failing over
    to one of the others if their 'usual' server was unavailable).   
More on
    this in my questions below. :-)

I have been scouring the Dovecot list, archives and Wiki and think I  
have come up with a set of issues we need to be aware of and things  
we have to do in order to make this work reliably.

Please would you let me know if I have missed or misunderstood anything?

Issues To Watch For
-------------------
*  For timestamp integrity our servers will be synched with NTP to our
    local time server.

*  Because sessions could be on different servers memory mapping of  
index
    files doesn't work well with NFS, so set
        mmap_disable=yes
   in dovecot.conf.

*  Dovecot relies on the mtime timestamp of mailbox files so the
    NFS-mounted mailstore needs to be mounted with these options on each
    server:
        actimeo=0
    (Are there any other mount options we should use too?)

*  We will be using filesystem quotas on users mailboxes.  We understand
    that Dovecot's index files are best on no-quota filestores so  
will store
    these separately.

Questions
---------
Q1. Would it be better to store the index files on NFS-shared  
filestore and
     direct users to any of the IMAP server machines?  Or to store  
the index
     files on local disk and direct each user to their 'preferred' IMAP
     server machine?

Q2. Does Dovecot (or "something") clean out old index files that haven't
     been accessed for a while?  Eg, when a user has temporarily come
     through on a different IMAP server to normal.  Or do the index  
files
     sit there untouched for evermore?

Q3. Storing the index files on the NetApp filer would give us the  
ability
     to share them between servers and grow their volume as need be,  
but at
     the cost of performance.  How big do the index files get?  Are they
     typically a few kBytes per message? Per mailbox? Per user? Or more?

Q4. We will be using Exim as the MTA, which can deliver direct to  
Maildir
     mailboxes.  However I understand that Dovecot's "deliver" LDA  
adds the
     benefit of updating the index files as each message is  
delivered.  Is
     this a significant gain?  Or is there little difference in  
actual use?

Q5. We have around 20,000 mail accounts and will therefore be seeing  
lots
     of concurrent IMAP sessions, usually secure (SSL) ones.  I have  
seen
     mention that this can give rise to "Too many open files" errors  
under
     Solaris.  How do we avoid this when we are likely to have several
     thousand concurrent IMAP sessions per server machine?

With many thanks for your time and advice...

Cheers,
Mike Brudenell

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