The Exim FAQ

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Q1001:  I'm running a large mail server. Should I set split_spool_directory to improve performance?

A1001:  Splitting the spool directory has most benefit if there are times when there are a large number of messages on the queue. If all mail is delivered very quickly, and the queue is always less than, say, a few hundred messages, there isn't any need to do this. With larger queues, there is a definite performance benefit to splitting the spool. It shows up earlier on some types of filing system, compared with others.

Exim was not designed for handling large queues. If you are in an enviroment where lots of messages remain on the queue for long periods of time, consider implementing a back up host to which you pass these messages, so that the main host's queue remains short. You can use fallback_hosts to do this, or a router that is conditional on $message_age.

Q1002:  How well does Exim scale?

A1002:  Although the author did not specifically set out to write a high- performance MTA, Exim does seem to be fairly efficient. The biggest server at the University of Cambridge (a large Sun box) goes over 100,000 deliveries per day on busy days (it has over 20,000 users). There was a report of a mailing list exploder that sometimes handles over 100,000 deliveries a day on a big Linux box, the record being 177,000 deliveries (791MB in total). Up to 13,000 deliveries an hour have been reported.

These are quotes from some Exim users:

"... Canada's largest internet provider, uses Exim on all of our mail machines, and we're absolutely delighted with it. It brought life back into one of our machines plagued with backlogs and high load averages. Here's just an example of how much email our largest mail server (quad SS1000) is seeing ... " [230,911 deliveries in a day: 4,475MB]

"... Exim has to ... do gethostbyname()s and RBL lookups on all of the incoming mail servers, and he runs from inetd (TCP Wrappers connected). All the same, it seems to me that he runs as fast as lightning on our SCO 5.0.4 box (1 Pentium 166) - far faster than MMDF which I (and many customers) had before."

"On a PII 400 with 128M of RAM running Linux 2.2.5, I have achieved 36656 messages per hour (outgoing unique messages and recipients). For about a 5 minute period, I was able to achieve an average of 30 messages per second (that would be 108000 m/hour)! We are using: (options that make a difference):

   queue_run_max = 1
   remote_max_parallel = 1

We have a cron job hat runs every five minutes that spawns 5 exim -q if there are less that 120 exim processes currently running. We found that by manually controlling the concurrency of exim -q processes contending for the spool for remote_smtp delivery that we gained considerable performance - 10000 m/hour."

Q1003:  We have a large password file. Can Exim use alternative lookups during delivery to speed things up?

A1003:  If you are using FreeBSD, this problem should not arise, because it automatically uses an indexed password file. In some other operating systems you can arrange for this to happen too. On Linux, for example, all you need to do is

   # cd /var/db
   # make

and put db before files in any /etc/nsswitch.conf lines you want to use db for.

On systems that do not include support for indexed password files, you can build one yourself, and reference it from the Exim configuration. For example, for routing to local mailboxes you could use this:

     driver = accept
     condition = ${lookup{$local_part}cdb{/etc/passwd.cdb}{yes}{no}}
     transport = local_delivery
     user = ${extract{1}{:}{${lookup{$local_part}cdb{/etc/passwd.cdb}}}

This assumes a cdb version of the password file.

Q1004:  I just wondered if it might be helpful to put the hints database on a RAM disk during regular operation. Did anybody try that yet?

A1004:  A user reported thus: ``I have found that this works great under Solaris. Make a RAM disk partition and keep everything in the db directory on it. However, when I try the same thing on Linux, I don't see the same boost. I think that Linux's file buffer cache works about the same. Plus, this leave more room for processes to run.''

There have been other reports that Linux's delayed buffer write provides better overall performance in general.

Apparently there is support in the Solaris kernel for a delayed writing, as in Linux, but Sun's server policy is to have it disabled so that you don't lose so much if the server crashes. There is a program called fastfs to enable and disable this support. You have to download and compile it yourself; find it by looking for fastfs.c in a search engine. Solaris performance is reported to be much improved, but you should take care to understand the potential hazards. In particular, fsck may be unable to “fix” disks automatically after a crash.

Q1005:  A lot of incoming mail is pushing up my system load too much, and there are many Exim processes. How can I control this?

A1005:  Have you set any of the Exim configuration options that limit what it does under high load? For example, queue_only_load, deliver_queue_load_max? See the list in the section entitled Resource control in the manual.

It sounds like a lot of simultaneous incoming mail pushes your system into uncontrolled overload. The multiple Exim processes are probably just multiple incoming messages. You can use the exiwhat utility to confirm this.

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