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@uoregon.edu