dovecot - Jan 2012 - Director questions

In playing with dovecot director, a couple of things came up, one
related to the other:

1) Is there an effective maximum of directors that shouldn't be
exceeded? That is, even if technically possible, that I shouldn't go
over? Since we're 100% NFS, we've scaled servers horizontally quite a
bit. At this point, we've got servers operating as MTAs, servers doing
IMAP/POP directly, and servers separately doing IMAP/POP as webmail
backends. Works just dandy for our existing setup. But to director-ize
all of them, I'm looking at a director ring of maybe 75-85 servers,
which is a bit unnerving, since I don't know if the ring will be able
to keep up. Is there a scale where it'll bog down?

2) If it is too big, is there any way, that I might be missing, to use
remote directors? It looks as if directors have to live locally on the
same box as the proxy. For my MTAs, where they're not customer-facing,
I'm much less worried about the latency it'd introduce. Likewise with
my webmail servers, the extra latency would probably be trivial
compared to the rest of the request--but then again, might not. But
for direct IMAP, the latency likely be more noticeable. So ideally I'd
be able to make my IMAP servers (well, the frontside of the proxy,
that is) be the director pool, while leaving my MTAs to talk to the
director remotely, and possibly my webmail servers remote too. Is that
a remote possibility?

On 23.1.2012, at 21.13, Mark Moseley wrote:
> In playing with dovecot director, a couple of things came up, one
> related to the other:
> 
> 1) Is there an effective maximum of directors that shouldn't be
> exceeded? That is, even if technically possible, that I shouldn't go
> over?
There's no definite number, but each director adds some extra traffic to
network and sometimes extra latency to lookups. So you should have only as many
as you need.
> Since we're 100% NFS, we've scaled servers horizontally quite a
> bit. At this point, we've got servers operating as MTAs, servers doing
> IMAP/POP directly, and servers separately doing IMAP/POP as webmail
> backends. Works just dandy for our existing setup. But to director-ize
> all of them, I'm looking at a director ring of maybe 75-85 servers,
> which is a bit unnerving, since I don't know if the ring will be able
> to keep up. Is there a scale where it'll bog down?
That's definitely too many directors. So far the largest installation I know
of has 4 directors. Another one will maybe have 6-10 to handle 2Gbps traffic.
> 2) If it is too big, is there any way, that I might be missing, to use
> remote directors? It looks as if directors have to live locally on the
> same box as the proxy. For my MTAs, where they're not customer-facing,
> I'm much less worried about the latency it'd introduce. Likewise
with
> my webmail servers, the extra latency would probably be trivial
> compared to the rest of the request--but then again, might not. But
> for direct IMAP, the latency likely be more noticeable. So ideally I'd
> be able to make my IMAP servers (well, the frontside of the proxy,
> that is) be the director pool, while leaving my MTAs to talk to the
> director remotely, and possibly my webmail servers remote too. Is that
> a remote possibility?
I guess that could be a possibility, but .. Why do you need so many proxies at
all? Couldn't all of your traffic go through just a few dedicated
proxy/director servers?