On 10/28/2011 6:35 AM, Arlin wrote:

How can we horizontally scale Dovecot across multiple servers? Do we require to install independent instances of Dovecot on each server?

The performance limiting factors for IMAP servers are disk IO, memory size, network bandwidth/latency, and processor power, usually in that order. To scale Dovecot horizontally would imply multiple servers each with independent storage handling a subset of the total concurrent user load.

We are planning to use a NAS/SAN device using ZFS or EFS for email storage. Each logical unit will be of 10TB and similarly as the no: of user increases we are planning to add multiple 10TB units.

This does not describe horizontal maildir performance scaling. This simply describes vertical scaling of the IO, to a point, as the nodes are all attached to one controller with finite IOPS performance, which is dictated by the speed of the IO ASIC and cache memory bandwidth WRT writes, and aggregate spindle IOPS WRT reads.

With this setup, you simply gain multi-node redundancy of the front end servers. However, you will *lose* performance compared to a single equivalent large host due to NFS/cluster filesystem overhead, latency, locking, etc, and your management/troubleshooting burden will be increased.

If your goal is simply increased performance and storage expandability, go with a single large host system. I'd only go the cluster route if you believe you need the redundancy.

-- Stan