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<title>The Exim FAQ Section 10</title>
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<h1>The Exim FAQ</h1>
<a href="FAQ.html#TOC">Contents</a>
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<h2><a href="FAQ.html#TOC289">10. PERFORMANCE</a></h2>
<p>
<a name="TOC290" href="FAQ.html#TOC290">Q1001:</a> I'm running a large mail server. Should I set <tt>split_spool_directory</tt> to
improve performance?
</p>
<p>
<font color="#00BB00">A1001:</font> 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 file system, compared with others.
</p>
<p>
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
<tt>fallback_hosts</tt> to do this, or a router that is conditional on
<i>$message_age</i>.
</p>
<p>
<a name="TOC291" href="FAQ.html#TOC291">Q1002:</a> How well does Exim scale?
</p>
<p>
<font color="#00BB00">A1002:</font> 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.
</p>
<p>
These are quotes from some Exim users:
</p>
<p>
"... 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]
</p>
<p>
"... 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."
</p>
<p>
"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):
</p>
<pre>
queue_only
split_spool_directory
queue_run_max = 1
remote_max_parallel = 1</pre>
<p>
We have a cron job hat runs every five minutes that spawns 5 <tt>exim -q</tt> if
there are less that 120 exim processes currently running. We found
that by manually controlling the concurrency of <tt>exim -q</tt> processes
contending for the spool for <b>remote_smtp</b> delivery that we gained
considerable performance - 10000 m/hour."
</p>
<p>
<a name="TOC292" href="FAQ.html#TOC292">Q1003:</a> We have a large password file. Can Exim use alternative lookups during
delivery to speed things up?
</p>
<p>
<font color="#00BB00">A1003:</font> 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
</p>
<pre>
# cd /var/db
# make</pre>
<p>
and put <tt>db</tt> before <tt>files</tt> in any <i>/etc/nsswitch.conf</i> lines you want to
use db for.
</p>
<p>
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:
</p>
<pre>
localuser:
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}}}</pre>
<p>
This assumes a cdb version of the password file.
</p>
<p>
<a name="TOC293" href="FAQ.html#TOC293">Q1004:</a> 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?
</p>
<p>
<font color="#00BB00">A1004:</font> A user reported thus: ``I have found that this works great under Solaris.
Make a RAM disk partition and keep everything in the <i>db</i> 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.''
</p>
<p>
There have been other reports that Linux's delayed buffer write provides
better overall performance in general.
</p>
<p>
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
<i>fastfs</i> to enable and disable this support. You have to download and
compile it yourself; find it by looking for <tt>fastfs.c</tt> in a search
engine. Solaris performance is reported to be much improved, but you
should take care to understand the potential hazards. In particular,
<i>fsck</i> may be unable to “fix” disks automatically after a crash.
</p>
<p>
<a name="TOC294" href="FAQ.html#TOC294">Q1005:</a> A lot of incoming mail is pushing up my system load too much, and there
are many Exim processes. How can I control this?
</p>
<p>
<font color="#00BB00">A1005:</font> 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 <i>Resource control</i> in the manual.
</p>
<p>
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 <i>exiwhat</i> utility to
confirm this.
</p>
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