> Larger ISPs restrict the size of user mail boxes. Most users don't keep
> too much email in their mail boxes, though, because it slows down their
> clients. :)
Quotas can be imposed at two levels, one in the MTA (exim) the other in the
OS. The overheads in implementing this in exim can be prohibitive to sites
that handle a lot of mail (we handle about 16 gigs). Leveraging the OS to
provide this functionality makes more sense but does require your MTA (exim)
to trap the write error and return a sensible bounce (bounces saying 'Can't
write to mail drop' look bad). Quotas make sense both with POP3 and IMAP4
since both RFCs now allow mail to be left server side after it is read (UIDL).
IMAP certainly has a lot to offer but for large ISP's the server software just
isn't up to scratch (not implying that most POP3 servers are !).
> > The other problem we have is that users configure their POP agents to
> > poll the server at ridiculously short intervals, like every 30 seconds,
> > so it is doing password checks at that rate. I guess you don't get that
> > problem on dialup connections.
This is a signifigant problem for ISPs even if they only support dial-up connections,
since there seems to be a critical mass of customers for any POP server that causes
it to trash when it is hit often enough. Round Robin DNS with multiple POP servers
makes sense here using NFS or some other type of common data storage.
>
> That's a good question. I will use a POP3 proxy that does the
> authentication in the near future. Right now, it performs between 20-30
> sessions (user, pass, stat, quit) per second, of which only stat and quit
> are handed to the real server. If needed, DNS RR sets could increase
> that number, using more machines.
Ditto.
>
> Can anybody provide some data on typical POP3 request rates per user
> with larger dialup installations?
>
About 102,000 requests per day for a user base of 40,000 customers.
> While looking at performance issues: With a Pentium II running at 450
> MHz, 512 MB RAM and a RAID configured as 0/1 with 6 disks, using Linux
> 2.0.35, I only get Exim to accept mail with a rate of up to 30-40 emails
> per second. Why? I wrote a few lines C to flood a server with SMTP
> requests, in case anybody is interested in profiling.
>
This is no mean amount of mail, at 30 mails per second you can handle
2592000 mails a day ! Also remember that you must factor in the processing
overhead needed to open/wait/close 30-40 TCP connections a second, adding another
NIC may improve performance.
> Just guessing, pre-forking Exim processes a la Apache might help to
> increase that rate, but I neither have the time for profiling nor
> hacking Exim.
>
--
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Conor Mc Goveran, conor@???
Systems Operator,
Telecom Internet http://www.tinet.ie
--
*** Exim information can be found at
http://www.exim.org/ ***
