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. 
