[Dovecot] Significant performance problems

[Brad Davidson]

Chris,

> -----Original Message-----
> Subject: [Dovecot] Significant performance problems
> 
> I'm sure my issues are a result of misconfiguration, but I'm hoping
> someone can point me in the right direction. I'm getting pressure to
> move us back to GroupWise, which I desperately want to avoid :-/
> 
> We're running dovecot 1.2.9 on Ubuntu 10.4 LTS+postfix. The server is
a
> VM with 1 vCPU and 4GB of RAM. We serve about 10,000 users with
anywhere
> from 500-1000 logged in at any one time. Messages are stored in
Maildir
> format on two NFS servers (one for staff, the other for students).

Is the webmail interface and imap proxy also running on this server?
What does memory utilization look like on the server? How much is being
used by applications, and how much is free for filesystem cache? What
mount options are you using on your NFS exports (on the NFS client
side)?

We run 60k accounts with about 10k concurrent sessions across 12
servers. Each server has 4 cores and 8GB of RAM, and mounts 16 NFS
exports spread across two servers. The servers handle close to 1k
concurrent sessions each without breaking a load of 1.

The keys seem to be keeping NFS IO latency down, and allowing the server
to cache as much as possible. If the Dovecot server is always having to
go back to NFS for client data, and the NFS server doesn't have enough
memory to cache filesystem metadata and/or spindles to access the data
in a timely manner, you're going to hit a pain point pretty quick.

Try bumping up the RAM on both servers to 8+GB, and make sure that you
don't have any mount options that would prevent the client from caching
data - noac for example is a killer. You could also try mounting with
noac, and disabling or turning down speculative readahead on the NFS
server. Have you followed all of your storage vendor's block alignment
guidelines when setting up the LUNs and virtual disks?

-Brad

---
Brandon 'Brad' Davidson
Virtualization Systems Administrator
University of Oregon Information Services
(541) 346-8098 brandond at uoregon.edu