And uptime is important, so you want to have that secondary running and ready, with a proxy in front of everything so you can switch as soon as you detect a failure. That's three hosts, plus your alerting has to be separate too, so that's four. Now, to orchestrate all this, we'll first get out Puppet...
If you're going from one machine to two, and you add an automatically failover mechansim, chances are your load switching mechanism is going to cause more downtime than just running from your single machine, and manually switching on failure (after being paged).
