yes its true MS doesn't support VSS snapshots on host-level. However, it's absolutely possible to create in-guest VSS of the filesystem with shared VHDX. We have used in-guest snapshots for years on our fileserver-cluster. Several months ago we moved that from physical machines to Hyper-V guests with virtual HBA's. Still with VSS enabled and working fine. I recently moved the SAN LUN's to shared VHDX files, to prepare for V8's somehow-ability to backup them. VSS however still works and is perfectly supported - wouldn't make sense to have a file-cluster that doens't support VSS; inaddition, in-guest VSS ofcourse stays inside the vhdx file - or on the clustered disk in this instance.
To prove my 'theory' what I did is the following:
on a testcluster I created a few in-guest snapshots of a clustered fileshare-disk that's a shared vhdx. I can make snapshots inside the guest just fine:
Creating snapshots on hyper-v host level is not possible. Creating a snapshot on the storage though (which could be done on a OffHost proxy) is ofcourse possible. The VHDX in that snapshot is not consistent ofcourse, but the in-guest snapshot inside that VHDX is. I mapped the storage snapshot to a machine here and can access that just fine after assigning it a driveletter for convenience. The Veeam_cluster_test.vhdx is the actual shared vhdx:
Then I copied the VHDX file to a local disk. Powershell doesn't seem to support mounting a VHDX on write-protected media, even if I supply the -readonly switch. That's not an issue for Veaam though, as the off-site proxy has read-only access to the snapshots anyway and can access the file.
Mounting the image file and listing the snapshots inside that vhdx:
Then using a regular cmd prompt I use mklink to mount that snapshot and presto; I can access all snapshots in the VHDX like that, and could if I want even easily move between snapshots in that vhdx:
So while this is a very awkward way of doing things, it DOES show I can have consistent data INSIDE a vhdx using in-guest VSS and after that snapshots on the storage box. The VM does not have to power down, in fact it kept running while I made this test for this post. While I know my approach here is more on file level than on block-level, I would kill to see Veaam support backing up cluster disks - even if through file level. Clusters are not passé, in fact I really advocate the use of them to save on storage (DAG / mirrors / DFS all require storage * numbers of hosts) which is expensive for small companies like we are. However, you guys probably have insane knowledge of working on blocklevel, so maybe you can even extract whatever you need in block-level from the VHDX.
I have this test setup available at any time, might you want to see this just contact me. Glad to help if I can.