I'm currently planning a more fault tolerant setup for my organizations Mail setup. We currently have two machines available in different datacenters, that we're planning to use for this, one being a physical host, the other being an lxc "VM".
The plan is to make the Physical host our primary Mailserver (lets call it server1) and the VM (lets call that server2) the secondary via DNS, so that when server1 goes down, server2 could take over until we've got the other one up and running again.
As with lxc I can't do fancy stuff like using DRBD or even GlusterFS due to permission issues, my choices for replicating data are rather limited.
As I personally like dbmail I've thought about using that with Postgres behind it to take care of storage. So basically I'm thinking of having dbmail with its own postgres running on server1, as well as another dbmail and another postgres on server2.
The question is: What options are there to keep those two postgres servers in sync without fancy stuff like drbd or a distributed filesystem, so that, when server1 goes down, server2 just continues where server1 stopped.