[imp] High Availability / Load Balancing

[Lee]

> 2) why bother replicating the sql database when you've still got a 
> single point
> of failure on the imap server?  mailstore is notoriously difficult to 
> replicate
> in realtime good enough to have a hot failover (if anyone does this 
> successfully
> , PLEASE speak up and share!!).  so you put the sql db on the same 
> server/tier
> as the mailstore and accept two single points of failure, both 
> catastrophic.
>

We divide our users over two-box cyrus imap clusters. Each segment of 
our users resides on a primary box with a second hot spare waiting to 
take over in case of failure. Each box has a live copy of the data 
using drbd. The boxes then monitor each other using linux-ha's 
heartbeat. We havent had any problems with this setup thus far. We use 
a dual perdition cluster to make the individual segments appear to be 
one single server.

L