Eric Horschmann of VMware recently posted on his blog about the ESX memory overcommitment feature. It can be a utilization benefit in some use cases, especially with lightly used virtual desktops. But Eric describes it as if it's somehow a game-changing economy.
The test he uses to support the claim is very impressive - if what you want to do is to power on virtual machines. If you're going to look at their screensavers all day while you do your work with a pencil and paper and abacus, power-on statistics are meaningful. And the moment you power on is the time you get the most out of page-sharing: nearly all pages are either operating system and services code pages (which are identical from guest to guest in many cases) or all-zero (which are all initially mapped to the same physical page).
Unfortunately for this scenario, I like to use my computer. I may push it a little further than some, but... I currently have a 20000-message Outlook mailbox and a 25000-message Thunderbird mailbox open, a 60-page Word doc, a 260-page PDF, five different browser tabs with graphics-intensive web pages... and, oh, yeah, I'm playing music in iTunes too. Not so much page sharing going on any more - in fact, I'm using 2GB on my 2GB notebook pretty consistently. And since now only 15% or so of my machine is running pages that it has in common with other people's machines - unless, of course, our tastes in music and our correspondence are identical - well, how do you think all that page sharing is really working out?
VMware, XenServer, ESX, Memory, Server Virtualization