If you have been using VMware ESX, then you might be a little spoiled by VMFS, VMware’s clustered filesystem. I have a hunch that many of us just take it for granted: create large LUNs, add to multiple ESX servers, create your virtual machines and be done with it. And, oh, migrate the VMs around at will with zero downtime thanks to VMotion.
Have you ever wondered what life in the virtualization world would be like without a clustered filesystem? Thanks to Microsoft Hyper-V, NTFS, and their failover cluster service, you no longer have to wonder, you can read about it in great detail.
In a world without VMFS, in order to have HA virtual machines or quick migration (let’s not bring up live migration today, since it is shipping “in 2010”) here is the most important point: Each virtual machine must be on its own LUN. Are you on good terms with your SAN administrator? Looks like the two of you are going to be chatting quite often.
Now if you prefer, you could set up several virtual machines on one LUN. But it turns out that such a plan would be destined for failure. No pun intended. Please take a look at the following quote from a Microsoft Technet blog:
If you group clustered VMs on a single LUN, don’t shut down the OS from within the OS or from Hyper-V Manager, this is not a cluster-aware shutdown and counts as a failure. The OS will be restarted by the cluster and then if you go “what the ?!?!” and shut it down again, depending on how the cluster resource is configured this will induce a failover to another node, taking the rest of your VMs with it. Try explaining that outage to your boss when a business critical server goes out of action for a couple of minutes at a crucial period because you shut down a scratch VM you had been using to test some software. I for one do not feel like having to get a change request authorized just to shut down a machine. Once you have made a VM a clustered VM, ALWAYS use Failover Cluster Management or the SCVMM Console to shut it down.
Ouch! What happens on Patch Tuesday? Let’s not find out.
One last thought: What about SAN deduplication technology that operates per-LUN?