Cluster Shared Volumes (CSV) is the new Microsoft Hyper-V R2 feature that allows virtual machines to be saved as plain text files and then imported into a spreadsheet or database for editing. Wait… that’s a different kind of CSV.
Actually, CSV is a layer on top of NTFS shared storage that provides some of the functionality of a cluster filesystem — multiple hosts can access a single LUN simultaneously. No more one VM per LUN jokes, please.
Unlike VMware VMFS, CSV relies on a single coordinator node for all metadata updates to the LUN — interesting.
The Secret Sauce
It turns out that CSV is a delicate feature and files on such a volume should never be managed directly. In an effort to protect Hyper-V administrators from themselves, Microsoft has taken an interesting non-technical approach to preventing CSV misuse.
After installing Windows Server 2008 R2, configuring storage LUNs, enabling Failover Clustering, and adding cluster nodes, CSV must be enabled on the cluster. In the process of doing so, a dialog box is presented that looks much like an end-user license agreement:
The text inside the box states the following:
The Cluster Shared Volumes feature is only supported for use with Windows Server 2008 R2 Hyper-V role. Creation, reproduction and storage of files on Cluster Shared Volumes that were not created by the Hyper-V role, including any user or application data stored under the ClusterStorage directory of the system drive on every node, are not supported and may result in unpredictable behavior, including data corruption or data loss on these shared volumes.
In order to proceed, administrators much check a box indicating that they have read the notice. Note: If you are the one that checked the box in your organization, please be sure to pass the warning on to your coworkers.
What about backup (“reproduction”) of virtual machines stored on the CSV LUNs? You cannot simply take backups of VM files like you would on a normal volume. In fact, if you attempt to back up any files on a Cluster Shared Volume by using the native Windows Server Backup tool, the following error is thrown:
Which leaves the Hyper-V administrator with just one option at this point: in-guest backups only. Eventually, System Center Data Protection Manager will support backing up Hyper-V VMs, but not right now.
Check out this hilarious ZDNet article bashing VMFS — they didn’t happen to mention any of these CSV advantages.
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