Monday, May 28

Cyrus IMAP file system tuning

Been busy, not enough blogging, bla bla bla. I know.
Just a lot of stuff going on at work at the moment, mergers and integrations.

We've had some problems with one of our IMAP servers at work running Postfix and Cyrus IMAPd. A for the job quite well speced machine, dual Xeons and 3x146Gb disk in RAID5 (4x146 in RAID10 would have been nicer). Anyway, the machine has got 50 or so IMAP users and perhaps 100Gb spool data on a ReiserFS partition.
The machines' avg. load has peaked at over 6.00 with about 75% in iowait on a bad day. I suspect that modern fancy e-mail search tools are to blame for the problems, applications building search indexes and such (Apple Mail anyone?).
Monitoring of the server showed quite a lot of inode update activity, even though there isn't *that* much new email coming in.
Must be our old (not so) dear friend atime that's making a little mess, I've used the noatimea and nodiratime mount options in the past with great success. Seen performance improvements of a couple of percent.
The mount-options noatime and nodiratime simply disables the feature to update the access timestamp of a file (and directory). I.e. when someone clicks and reads an email in their mail application the inode atime timestamp is updated. When is this atime timestamp used? Never.
Did a quick online remount of the spool fs with noatime and nodiratime.
The result?
Avg. load hasn't touch 1.00 since. Wow! I was expecting an improvement, but not that big.
Great and easy way to improve performance on IMAP spools.
mount -o remount,noatime,nodiratime /var/spool/imap
And don't forget to update /etc/fstab with the same mount options.


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