[Dovecot] nfs director

[Timo Sirainen]

On 26.8.2010, at 23.54, 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).

> I fail to see advantage if anything it add in more point of failure, with
> several hundred thousand user, we can ill afford to mess around or add to
> complexity, sometime keeping it simple is simply way to be, when use
> qmail/vpopmail, we never had one failure or problem, ever, we very proud of
> this record so our users.

Are you using Dovecot? For how long? Do you see any errors in your logs?

> if director service assign 60K user to each front end,  how it handle if 5K
> simultaneous user login, but all 5K happen to be assign to that one machine,
> it do all work whilst other 7 server sit there do nothing negating what the
> LB is design for?

The users are distributed based on the MD5 hash of their username and that gives a pretty good distribution of where the users go. Unless 5k of your users suddenly decide to coordinate such attack, I doubt you'll ever see anything even close to that happening. But sure, there is some variation. I don't have any real world numbers, but my guess is it's normally less than 20%.

Also with some more work it would be possible to dynamically adjust how many new users are getting assigned to servers, but I wasn't planning on implementing that unless it becomes clear that it's needed.

> Is it really worth it? Do we really need this, or just let foundry switch
> handle it as it does now.
> We also have 24 front end SMTP server, these deliver mail to netapp filer,
> all 24 plus 8 pop3 server and 2 webmail imap server all mount /vmail, so all
> access same maildir. it seem work very effective thus far and for many many
> year when we use qmail and vpopmail.


If you don't mind random Dovecot errors about index corruption I guess you're fine with how it works now. I guess your mails are delivered to maildirs by qmail? If you ever switch to Dovecot LDA you'll probably start getting more errors. And if you ever plan to switch to dbox format then at latest you'll need director.