TLDR: Newer versions of Windows will free blocks on a Cluster Shared Volume.
I work on a lot of Failover Clusters and about once a year it would come up that a customer had a situation with disk space. Usually it was older, out of support, versions of Windows and the scenario was that recently a bunch of VMs were moved from 1 Cluster Shared Volume (CSV) to another. The CSV volume on the FlashArray was not releasing the blocks. That left a customer with a handful of choices:
There are many different strategies for backing up a production Hyper-V environment. In this blog post I will focus on a hardware Volume Shadow-Copy Service (VSS) snapshot of a Cluster Shared Volume (CSV). VSS creates an application consistent snapshot where all of the VMs are in a good state so that if you restore the volume, or clone the snapshot and mount it outside of the Failover Cluster, the VMs will boot without issue. Keep in mind that this is a hypervisor driven snapshot, which is different from an application snapshot inside of a virtual machine. An Exchange Server or SQL Server database will not show that a ‘backup’ has been taken. Some third party backup applications (VSS Requestors), can handle this, but that is because the backup application is performing additional work inside of the VMs.