I've recently been asked to build a redundant mailstore, using two server-class machines that are running Ubuntu. The caveat, however, is that no additional hardware will be purchased, so this rules out using any external filestorage, such as a SAN. I've been investigating the use of DRBD in a primary/primary configuration, to mirror a block device between the two servers, and then put GFS2 over the top of it, so that the filesystem can be mounted on both servers at once.
While a set-up like this is more complex and fragile than using ext4 and DRBD in primary/secondary mode and clustering scripts to ensure that the filesystem is only ever mounted on one server at a time, it's likely that there will be a requirement for GFS on the same two servers for another purpose, in the near future, so it makes sense to use the same method of clustering for both.
The following guide details how to get this going on Ubuntu 10.04 LTS (lucid). It won't work on any version older than this - the servers that this is destined for were originally running 9.04 (Jaunty), however, I've tested DRBD+GFS on that release, and there's a problem that prevents it from working. As far as I'm concerned, production servers should not be run on non-LTS Ubuntu releases, anyway, because the support lifecycle is far too short. This guide should also work fine for Debian 6.0 (squeeze), although I haven't tested it, yet.