I'm considering starting to use H-Sphere. How easy is it to expand a cluster? For example, how easy is it to switch a service (control panel, mail, apache, dns or mysql etc) from an old server to a new server with a totally new IP?
Is each user assigned to a specific servers in a cluster, or are they spread... for example by sharing a customer's data on a network drive with two or more MySQL servers to share the load?
In a nutshell: it's a pain but doable. Above all, don't give yourself more grief than you have to, and give every service its own IP address in advance. This is 20/20 hindsight speaking.
H-Sphere recognises two key types of servers: physical (the actual box) and logical (the service). Treat the logical servers as though they were physical ones.
Each user is assigned to a specific web server, and all sites under that account are on the same server. This is pretty much set in stone and you don't want to move people around anyway. Individual sites are assigned to single mail and SQL servers and a single set of DNS servers but not necessarily to the same ones as the other sites in the account.
It's not that difficult if you know SQL and can follow the design of the central database, we've done more IP migrations than I care to remember and with a bit of SQL knowledge it's not that difficult - although I wouldn't want to do it every day.