I wouldn't say that we have an equation, per se, but it is true that we have a reasonably good idea of how many people we can host on a given piece of hardware before problems develop. A number of criteria go into this, ie. what plans people are on, how many 'CPU minutes' they use, etc. While it's not as scientific as you might think, we've gotten fairly good at it. Beefier servers host more people, so it all works out.
It is a bit cost prohibitive to replace servers on a regular basis when older servers are perfectly functional and perform well. It'd be unreasonable, I think, to expect these servers to be replaced as new hardware becomes available unless they're broken - at least, at the prices we offer.
Plus, by being "old" such servers are probably among our most stable and reliable servers. Such servers already have a reliable 'mix' of sites/customers known not to cause problems, as we long ago weeded out any troublesome accounts. In fact, I'd say that the least stable servers we have at any given point are probably new-ish ones. They get better over time as we identify problem areas and resolve them.
So, yeah, I wouldn't be so quick to jump onto new hardware if you haven't had trouble with the hardware you're on. I know that almost all of my personal sites are stored on a pretty old server, and I'm very happy with it. For quite a while, I even had sites on the very first server we ever had, and it too was pretty good (though admittedly it needed some upgrades over the years to keep up with what we put on it).
- Jeff @ DreamHost
- DH Discussion Forum Admin