[Dovecot] nfs director

[Timo Sirainen]

On 27.8.2010, at 1.47, Edward avanti wrote:

>>> Please can you explain why this is advantage over a hardware load
>> balancer.
>> 
>> It guarantees that the same user is accessed via the same server. Hardware
>> LB can at best assign the connections from the same IP to the same server
>> (but not e.g. new mail deliveries, or if user has multiple clients like
>> home/work/mobile, or simply accesses webmail at the same time as has a
>> client open).
> But how does it help when new mail arrive on one of the 24 SMTP server,
> where there can be no direction that I can see. You might control where user
> login to get mail, but not which server accept mail.

If you're delivering mails using standard Maildir then yeah, it's ok that your 24 SMTP server writes it. If you touch any of Dovecot's index files then you can't do it on the SMTP server 100% safely. Instead you'd have to have your SMTP server deliver it to the correct destination server using LMTP protocol via LMTP/director proxy.

> there is a reason we have 8 pop server, in evening, pop mail is very busy by
> many user, many server often have thousand of concurrent user session but
> not multiple session per user.

Yeah, with POP3 there aren't much problems since there is almost always only a single client. With IMAP it's different.

>> getting more errors. And if you ever plan to switch to dbox format then at
>> latest you'll need director.
> 
> So this all because of index?

Pretty much, yes. But also the dovecot-uidlist file that keeps the UID <-> filename mapping. That's less likely to break though.

> what is ramification for user if this corruption occur?

Random .. usually slight performance loss, maybe kicking out one user session but the next one working. But I can't guarantee that it won't be worse.

> is it self healing over new pop or delivery?

There have also been corruption bugs that Dovecot hasn't managed to fix automatically, but it's been a long time since those happened with maildir.

> Can index be disable?

In delivery time or everywhere? In delivery yes, you can disable indexes, but you can't disabled dovecot-uidlist updates and those can also break.

> We thought using dovecot deliver, but this seem not good idea now? As
> dovecot not know how qmail or postfix put mail in, since just put it in, can
> not dovecot deliver work same? or have only dovecot on pop3 server update
> index?

Yes, it would be possible to change Dovecot code so that deliver won't update dovecot-uidlist either, but that makes the code just more complex so I haven't wanted to waste my time adding workarounds for a few NFS users..