Re: [SM-USERS] web server capacity for squirrelmail

[John M. <jm...@iv...>]

> In our current situation, we have approximately 500 users, with up to
> 200 users at any one time hitting the web server.  Each apache process
> is using between 10 and 20MB of RAM each, and the CPU is usually 95%
> idle or higher.

Yeah, this is definitely an interesting situation.  I've got a rather
lengthy story about my college's run-in with this, but I'll try to
condense it down here...

We've got about 2600 users in 24 campuses across Indiana.  In short, we
never know exactly what's going to happen until it does. :)  SM is
providing us with excellent web mail, but who knows how well the load is
going to be handled when everyone gets migrated over?  This past Monday,
we found out - the hard way.

We were running Courier-IMAP, Postfix, PHP 4.1.2, SM 1.2.6 (+in-house
mods), Apache 1.2.3, and Linux (kernel 2.4.14), all on a dual PIII/1gig
with 1GB ram, 100GB scsi raid-5.  All went well on Monday until about
11am, when several hundred users decided to hit web mail at once.  I saw
the load avg go up to about 20 before the box became unusable.  Sometime
later on, it did a mostly-crash, and I ended up rebooting it.  Ew, I felt
so dirty...

Fortunately, we had planned for a rush of users moving from LookOut to web
mail, and had an LVS cluster (linuxvirtualserver.org) contingency plan. 
For anyone considering having lots of users, I highly recommend the
solution.  A single PIII/700 acts as the load balancer and failover
controller, 3 PIII/1gig+1GB Dell 1550's are the web "farm."  They NFS
export all of SM and the PHP sessions directory from the mail server
itself, then do the IMAP work to the mail server itself as well.

LVS works by transparently passing incoming tcp requests off to a cluster
of machines based on queuing decisions (round robin, weighted least
connections, etc.).  The whole system works quite well, and handles
several hundred users (over SSL) without so much as a hiccup.  We should
be able to easily scale to move all of our users to web mail, even if we
have to add more nodes to the cluster.

Points to remember:


- Lots of SSL is bad on your cpu's.
- Pick a good IMAP daemon.  Anything that uses mbox storage is going to
roll over and die very quickly.- Again, Maildir is good.  You can even serve it up over NFS if you need to.
- If you can, split the IMAP and web server load onto separate machines,
whether you're technically clustering or not.- If you're doing lots of virus scanning, mailing lists, or other apps on
your SMTP box, you may want to split that off from the IMAP server as
well.

Hope that helps...
   John




-- 
John Madden
UNIX Systems Engineer
Ivy Tech State College
jm...@iv...