It is just a couple of years since Microsoft announced its intention to seriously compete with VMware in the hypervisor market. Since then, Hyper-V has begun to move out of testing and in to production. Now Microsoft is looking to take it further.

When Microsoft first introduce Hyper-V it made a lot of very bold statements. By the time the product shipped these had been heavily watered down. Most customers were happy with the feature set but there was one feature that Microsoft has been pressed on consistently - live migration.

Microsoft had increasingly showed its irritation over the live migration arguing, with some justification, that when you look closely, there are few people doing automated live migration. This means that their staged migration approach is just as valid for customers.

Now, in an attempt to shut up its detractors, it has introduced live migration into Hyper-V 2 which will ship later this year with Windows Server 2008 R2. At the same time it has upped the game for migration, introducing a feature ahead of VMware that will allow it to put the pressure back on its main competitor.

That feature is Processor Compatibility. But what does it mean?

Many people think that you can simply migrate a VM from one server to another providing that they have the correct hypervisor on the server. Not true. Migrating VMs is never quite that easy. Moving from an Intel based server to an AMD based server is a problem. But so is moving from one Intel processor family such as Nehalem to another, such as Pentium 4.

It would be nice if all the machines in the datacentre were on the same processor release and family. Inside a lot of large virtual environments, there has been a move towards more standardisation of hardware but this is a hidden cost that belies the supposed flexibility and savings that virtualisation is supposed to offer.

As a result, you cannot simply live migrate a lot of VMs. This is because when you change any core bits, even with VMs, you still need some HAL components for the processors. If you are going to move VMs across processors, you need to shut down and then restarted the VM on the new hardware to get the right underlying drivers. Unsurprisingly, Microsoft has taken the opportunity to point to this as a reason why live migration is not widely adopted.

According to Jeff Woolsey, Principal Program Manager, Windows Virtualisation, to get around this, Microsoft has decided to hide some of the processor flags. In effect this normalises the processor features. An example is that Nehalem ships with a feature SSE 4.2 which isn't supported in earlier processors. This will only work in an Intel world (let's hope the EU doesn't find out) and only on VT enabled processors. However this does encompass four different Intel processor families which is a significantly large proportion of the Intel based servers in the market today.