One of the nice new additions in VMM 2008 R2 is Quick Storage Migration or QSM. This allows you to migrate a VM from one storage device to another, e.g. from one SAN to another, one LUN to another or from a per LUN installation to a CSV (Cluster Shared Volume).
Edwin Yuen (senior dude at Microsoft on virtualisation), talks about the mechanics of QSM in an excellent blog post. The short story is:
- A Hyper-V snapshot is taken by VMM 2008 R2 of the VM.
- This means our VM is running from AVHD’s, a form of differencing disk. All writes now happen to the AVHD’s.
- VMM 2008 R2 is free to copy the original VHD(s) to the new destination.
- The VM is put into a saved state and the AVHD’s are merged into the VHD’s at the destination. This takes probably around a minute, depending on how long the VHD copy took and how much data was written to the AVHD’s.
- The VM starts up on the destination, running only from the VHD’s.
There’s something to watch out for here in moving from “1 VM/1 LUN” installations.
Imagine you have a VM with 100GB of disk and 2GB RAM. How big do you think the LUN will be for that on a Hyper 2008 cluster? Probably only around 113GB ( (VHD+RAM)*1.1 = (100+2)*1.1 = 113 ). That’s because you can’t waste disk. You’ve just enough space for your VHD’s, the config files and a save state for the RAM. If you snapshot that VM it will (by default) create a snapshot in the same location. That AVHD will start out small but will grow. If the file VHD copy takes a while and there’s lots of write activity to the AVHD then there will be an issue. Your LUN will fill and your VM will pause during quick storage migration (QSM).
I don’t have the gear to test VMM 2008 R2 so I’m going to ask about this. VMM 2008 doesn’t have a field to specify an alternate LUN for snapshots but the Hyper-V console does. Maybe you could use that to relocate the snapshot AVHD’s in advance of a QSM from a per-LUN installation to a CSV? I suspect this should work … you can change that Hyper-V setting while a VM is running. It’s times like this I wish I had €20-€30K of gear to test with!!!!
EDIT #1:
I contacted Ben Armstrong (Virtual PC Guy) via the MVP newsgroups and he got onto the VMM team. The answer is "yes" to the problem and solution.