load balancing at fastmail.fm

[urgrue]

> If it's using block level replication, how does it offer instant 
> recovery on filesystem corruption? Does it track every block written 
> to disk, and can thus roll back to effectively what was on disk at a 
> particular instant in time, so you then just remount the filesystem 
> and the replay of the journal should restore to a good state?
Yes. I may be wrong but to my understanding at least NetApp has this 
capability.

>
> With file based replication, about your only way of failure is the 
> replication software going crazy blowing both sides away somehow, 
> which given that the protocol is strictly designed to be one way, 
> seems extremely unlikely that anything will happen to the master side.
I agree of course about avoiding SPOFs, but I do like a multi-tiered 
approach, I mean multiple lines of defense. I use SAN for its speed, 
reliability, and ease of administration, but naturally I replicate 
everything on the SAN and have "true" backups as well.