[K12OSN] Re: Server Tuning

[Les Mikesell]

On Thu, 2005-04-21 at 14:15, linuxk12 at mountainlake.k12.mn.us wrote:

> iostat is new to me.  I've been running it the last couole fo days
> with the "-x" paramenter to see all of the details.  It looks like
> the useful stats for me are "avgqu-sz" which I interpret to be the
> number of requests waiting in the queue to be served and "await"
> which is the number of milliseconds an average request has to wait
> to be served.

Note also that when you install sysstat package it sets up timed
statistics snapshots that you can examine with sar.  

> It looks like "await" is typically in the 50-100 range, but occasionally
> I see it at 30k (30 seconds assume, I since this is a measure of milli-
> seconds).  Sometimes this number is much much higher, but perhaps it
> is a problem with the software dealing with a counter turnover or something
> like that.

I once had a machine with a raid controller that would accept and queue
thousands of commands - it worked great under normal conditions but
once in a while it hit some kind of high-water mark in the queue and
would not accept anything else until most of the command queue was
completed, resulting in a several minute stall of the whole machine.

> I was not familiar with either of these IMAP/POP servers, so did some
> checking.  Cyrus appears to really shine if you have many more than
> 100 simultaneous users.  It uses its own database to manage email and
> can access it very efficiently.  Dovecot focuses on security and
> performance.
> 
> Dovecot looks easy to compile and install, so I have been playing with
> it a bit.  The security features are making it a pain to set up.  I've
> been running it in a debugger just to see what exactly is tripping it up.
> It's not quite a drop-in replacement for my current setup, so I'll have
> to set it aside for now.

What kind of problem are you having - and are you using Maildir or
mailbox format?

> One consideration is that the mailbox file space is on a different drive
> than the user file space, so imap was never in contention for disk
> access, only CPU cycles.

The high load average makes it sound like a CPU problem and Cyrus or
Dovecot would help by avoiding the index/sort on messages at each
mailbox connection.

-- 
  Les Mikesell
   les at futuresource.com