We are in the initial planning stage for a high-availability (HA) Linux server cluster which will offer e-mail services (SMTP and IMAP) for about five hundred users. To this effect, we will probably be using DRBD in a "shared nothing" configuration.
DRBD is a block device which is designed to build high availability clusters. This is done by mirroring a whole block device via (a dedicated) network.
Initial tests with a 400 GB disk slice look very promising indeed!