One of the greatest things about virtual machines is the ability to take snapshots, which can be used to quickly roll a VM back to a known state.
Previously, I have written about some of the shortcomings with Microsoft’s Hyper-V snapshots. Or were they checkpoints? I forget… but that’s not important. The main problem with the Hyper-V snapshot implementation — that does not affect VMware ESX — is that the snapshots are not actually removed until the VM is powered off. All the way off — reboots don’t count. And the VM must remain powered off until the AVHD file is completely merged, otherwise the process is aborted.
By the way, if you are using System Center Virtual Machine Manager 2008, you will have no way of knowing when the snapshot merge is complete. SCVMM just simply does not report this information — switch to the other single pane of glass (Hyper-V Manager) if you need to be sure. Hyper-V administrators are used to this by now.
So, if you find your VM in “Paused-Critical” state, you might want to check for a hidden snapshot file filling your LUN. If you’ve landed on VCritical in search of such a solution, you are not alone — quite a few Google searchers end up here due to this issue. Before Ben Armstrong’s recent — and very informative — series on snapshots, VCritical was among the first to elucidate this issue.
If a hypervisor’s snapshot functionality is not recommended for production, is that hypervisor itself ready for production?