http://blog.scottlowe.org/2008/02/20/a-few-thoughts-on-xen/

I read Scott's blog as often as I can - he's got a lot of experience in the technology industry, and a hell of a lot more than me around Virtualisation!  Scott's got a lot of experience with VMware, but as explained in the post (link above), he'd starting to play with Citrix's XenServer too.

The thing I find interesting about the post (and also the comments) are around the world of x64, and XenServer's native x64 hypervisor, compared with VMware's 32-bit offering.

"First, the XenServer folks had a lot to say about how their 64-bit hypervisor was better than VMware’s hypervisor because VMware’s product was “only” a 32-bit hypervisor"

One of the reason's I'm so interested in this post is that the Hyper-V hypervisor shares many traits and similarities with the XenServer hypervisor (a number of XenServer developers reside in Redmond...), however, I would never state that Hyper-V was 'better' than VMware because it was x64 and ESX wasn't.  For a 32-bit platform, ESX scales massively.  If I look at Windows Server 2008 32-bit version, there is no way, to my knowledge, it can support anywhere near the 256GB that VMware ESX 3.5 can support.  However, when you look at x64 Windows Server 2008, the amount of addressable memory scales to 2TB.  2 Terabytes.  That's a lot of RAM.  Are there many systems out there that are going to have 2TB, at least in the next year?  I doubt it.  The key thing is future proofing for me.  Going with x64 just gives us that platform for the future.  We can tick that box and say 'It scales'.  Now let's focus on the other features; something that VMware has done very well indeed.

It's not a case of x64 being better or not.  Just looking purely at the amount of addressable memory, comparing 256GB (ESX) with 2TB (Hyper-V) seems a huge jump, but to most people, even a box with 256GB is way beyond their needs for some time to come!  It's just when you start thinking about the future - 1, 2 years down the line, then perhaps 256GB might not be enough, who knows?  By then, I'm sure VMware will have brought out an x64 version that scales really well too :-).  Oh, and just to add, Hyper-V supports 64GB Guests too :-)

Anyway, I'll let you have a read of the post - there's a lot of interesting food for thought, especially in the comments.