> 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 !).
I thought about using OS quota, but I really do not want to create a UNIX
user for each POP3 user, so I use exim quotas. I already found out that
using Maildir, delivery gets very slow as the number of emails increases,
because exim stats all files to calculate the mail box size. I guess that's
something we have to live with. SMP machines will help to provide more CPU
power per machine.
> > 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.
Only 2.55 requests per day for each customer? That's good news.
> 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.
To me, that means: Indeed, not even 3 million emails per day, assuming
an even load distribution over time. Unfortunately, that assumption
does not hold and I don't want to have other systems retry often until
they finally get through during rush hours. I will use DNS RR sets for a
scalable input queue, but I still would like a higher rate for incoming
mail, because it would save hardware. I certainly hope that exim only
forks a process per email instead of using fork()/exec()?
Michael
--
*** Exim information can be found at
http://www.exim.org/ ***
