On Tue, 2005-08-16 at 09:42 +0100, Simon Chappell wrote:
> We currently have a Dual P3 700 with 6 hard disks with 600 accounts on
> it. We have moved webmail of to another server but Imap and Smtp is
> still pushing the 15min load average past 6.00 .
> We have about 100 users with Outlook using Imap and the rest use Horde
> webmail.
600 accounts is actually pretty small, although I don't have a feel for
qmail resource usage (I would expect it to be even harder on the disks
than most of the other MTAs due to its aliasing mechanisms, but
otherwise not dissimilar).
The major hit for any mail system is disk bandwidth - you are
fundamentally constrained by the number of I/O operations (in particular
writes) per second you can get committed to your disks, so a decent RAID
system makes a lot of difference, especially if you have proper write
through caching that works. Thats the case for both the MTA (it has to
satisfy conditions such as getting the message to stable storage before
you can acknowledge a message) and IMAP.
However nowadays the biggest CPU hog is content scanning.
It will be worth looking at comparisons of the IMAP systems and their
storage mechanisms - these will affect your disk (bandwidth)
requirements considerably.
Adding lots of RAM will help, but disk is the major factor. Choosing
your filesystem wisely will also help - although be aware that many of
the comparisons between filesystems are a little dated (ie ext3 on Linux
has directory indexes now - that makes a lot of difference and may make
the decision between bleeding edge features and stability easier. In
almost every case switching off atime updates is a big win.
There are a number of discussions on designing and scaling mail systems
in the archives. If anyone can dig those out (and put references into a
wiki article) then that would be useful.
Nigel.
--
[ Nigel Metheringham Nigel.Metheringham@??? ]
[ - Comments in this message are my own and not ITO opinion/policy - ]
