Miserable performance of cyrus-imapd 2.3.9 -- seems to be locking issues

[Henrique de Moraes Holschuh]

On Thu, 28 Feb 2008, Michael Bacon wrote:
> I've never seen drbd used for Cyrus, but it looks like other folks have 
> done it.  The combination of drbd+lvm2+ext3 might put you somewhere 
> unpleasant, but I'll have to let the Linux-heads jump in on that one.

Don't try it with 4k stacks, IMO.  It could blow up badly.  Stacked devices
and filesystems have this nasty tendency to eat up way too much stack :(

And whatever you do, don't do mailspool IO patterns over Linux raid5 with
the raid bitmap updates enabled and ext3.  Performance goes to crap.  I
don't exactly know how to enable or disable these bitmaps, though.  Look at
mdadm's manpage.

> > a linux software RAID 5 (3 SATA disks). On top of the md layer is the
> > drbd device; on top of that is an lvm2 logical volume; on top of that is
> > an ext3 filesystem, mounted
> > as '/var/imap'. The mail is then in /var/imap/mail and the metadata in
> > /var/imap/config (and we also have /var/imap/certs for the ssl stuff, and
> > /var/imap/sieve for sieve scripts).

Do look into that md raid bitmap option, remember that using lvm anywhere in
a chain kills any and all write-barrier support which means a full
sync-cache command to the HD even if it is a nice SCSI one, remember that
drbd is not a lightning bolt either (you do have a direct gigabit ethernet
link in use just for the drbd sync, don't you?), and remember to inform lvm
AND ext3 of the raid stripe size when making the filesystems and lvm
volumes.

Also, the usual mount tricks like noatime should apply.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh