Performance Notes
Performance issues for you to consider. If you never expect to have
more than 100 simultaneous users, chances are any hardware you have
will be fine. If you plan on having thousands or more users, please be
sure to review this section.
If your configuration directory is not /var/imap, adjust accordingly.
* /var/imap/proc - After a successful login, the imapd creates a file
in /var/imap/proc that is its unix process id. It also contains the
name of any SELECT'd mailbox. The file is deleted when the user
logs out.
Given the potential load, this is a good candidate to move
elsewhere. This can be done by symlink'ing the directory to another
partition. We symlink it to a directory on a memory/viritual memory
filesystem (specifically Solaris' tmpfs). If you use a tmpfs type
filesystem, make sure that you have sufficient memory/swap to do
this.
Some people don't care about this information and just #ifdef out
the code. We probably should add a configure option to do this.
* /var/imap/mailboxes.db - The mailboxes list is often the ultimate
source of contention between imap processes, especially if clients
are inefficient about their use of the LIST command. For this
reason it is often better to use the skiplist backend which is
optimized for enumeration of the database, as opposed to the
default, Berkeley DB (use --with-mboxlist-db=skiplist).
Mika Iisakkila (mika.iisakkila@pingrid.fi) writes: Nevertheless,
you can also tweak the Berkeley backend if you want to or have to
stick with it. Cyrus doesn't do anything to increase the BDB cache
size, and the default (256 kB) is way too small for any reasonably
large site. With some 50000 mailboxes and random operations, I
found the hit rate for default BDB cache to be 70-80%. After
growing the cache size to 2M, hit rate approached 99% and disk
traffic was greatly reduced since most of the operations are reads
anyway. Therefore processes could complete their work and release
their locks much more quickly, and the dreaded "DBERROR: xxx
lockers" messages stayed at a comfortable level. You can modify the
source (/lib/cyrusdb_db3.c, the setting is commented out) or you
can put a DB_CONFIG file under /var/imap/db with the appropriate
setting. Read more about this in the Berkeley docs before trying
it, typos and incorrect settings can cause havoc.
* /var/imap/deliverdb - Unless you disable duplicate delivery
suppression, each time a mail message is delivered it needs to lock
the database and check to see if the message-id has been seen
already. If you require really high throughput delivery, you may
want to disable this feature.
We run with it enabled and it doesn't significantly impact our
performance.
* /var/spool/mqueue - Sendmail can be pretty harsh on the spool
partition. Having this on a separate disk is usually a good idea.
Consider using LMTP and delivering from a separate machine.
* Unused SASL mechanisms - If you just build the SASL library and
copied all the mechanisms into /usr/lib/sasl2, the imapd will try
to use them and allocate some amount of memory. In general, the
operating system will swap out those pages but you may be
allocating more swap space than you need. So, look in
/usr/lib/sasl2 and if you don't plan on using those mechanisms,
don't leave them there.
* You may want to increase the listen queue value when starting up
the master process. For example, you may want to do this if you see
the Listen queue drop counter increasing quickly. For example,
under Solaris, look at the variable tcpListenDrop (from netstat -sP
tcp).
* Database recovery. If restarting the server takes a long time due
to the cyrusdb database recovery procedure (this is usually true if
you have a large number of deliveries) you should look into
shortening the interval between checkpoints, controlled by the
cyrusdb event in /etc/cyrus.conf. We run checkpoints every 5
minutes; the current suggested install interval is 30 minutes.
* Some filesystems support the noatime mount option. The server does
not use the atime information so you can go ahead and enable this
feature.
* Depending on your syslog configuration and usage volume, Cyrus may
generate thousands of syslog messages. On Linux, syslog performance
can be greatly improved by disabling synchronous logging (disabling
fsync() after each message). Prepending filenames in
/etc/syslog.conf with a "-", e.g., "/var/log/maillog" becomes
"-/var/log/maillog", disables syslog's fsync() call after each log
message. If you log many messages those fsync()s will kill your I/O
throughput. Note that if you do not need the detail provided by the
LOG_DEBUG level, then not logging these messages can significantly
reduces the amount of log entries that cyrus makes.
In general, there's no magic bullet for performance. It depends on your
hardware, your operating system, and how your users use the system. In
general, an imapd process takes up anywhere from 256 Kbytes of memory
to 512 Kbytes when it is first fired up. CPU has not been a big deal,
but it may become more important as the imap sessions are encrypted and
now that searching may be more frequent. Disk I/O is probably the most
important and having a hardware RAID subsystem with an amount of
write-back cache would be a good thing.
Finally, if you are talking about less than 100 interactive users it is
likely that any relatively modern hardware can support it. If you are
talking about having more than 1000 interactive users, you should know
how to predict your utilization, go overboard on hardware, be willing
to suffer growing pains, or be able to hire someone that can help.
There are a number of good performance tuning articles out for Solaris
by Adrian Cockcroft. Go to your favorite search engine and look for his
name.