Windows Server 2012 R2 adds support for ReFS with CSV. However, Hyper-V virtual
machines are not supported on ReFS, which means that you will still use NTFS to
store your active virtual machines. Windows Server 2012 R2 also adds extended
clustered storage space support, including support for storage spaces that use tiering,
write-back cache, and parity. Data deduplication is also supported with CSV in
Windows Server 2012 R2. While Windows Server 2016 adds Hyper-V support with
ReFS, this is only for Storage Spaces Direct deployments. Normal non–Storage Spaces
Direct CSV should use NTFS. A big reason for this is that ReFS does not handle Direct
I/O, and all I/O is therefore redirected at the filesystem to the coordinator node of the
CSV over the cluster network. This is why RDMA networking is recommended for
Storage Spaces Direct, and with Storage Spaces Direct everything is redirected anyway
since there is no shared storage. Ultimately, use NTFS.
A nice addition in Windows Server 2012 R2 is increased diagnostic capabilities for
CSV. You can see the reason a CSV is in redirected mode, which previously was hard to
ascertain. Using the Get-ClusterSharedVolumeState PowerShell cmdlet shows the CSV
state and the reason for the state. In the following example, all my CSVs are not block
redirected, but you can see where the reason for redirection would be displayed. My
disks are in filesystem redirected mode because I’m using ReFS and not NTFS to show
a type of redirection. However, typically you should always be using NTFS for CSV
unless you are using Storage Spaces Direct, as previously mentioned:
PS C:> Get‐ClusterSharedVolumeState
BlockRedirectedIOReason : NotBlockRedirected
FileSystemRedirectedIOReason: FileSystemReFs
Name : Cluster Disk 2
Node : savdalhv93
StateInfo : FileSystemRedirected
VolumeFriendlyName : VMs
VolumeName : \?\Volume{772694a4–985d-48ee-9902–104be6c64181}\
BlockRedirectedIOReason : NotBlockRedirected
FileSystemRedirectedIOReason: FileSystemReFs
Name : Cluster Disk 2
Node : savdalhv92
StateInfo : FileSystemRedirected
VolumeFriendlyName : VMs
VolumeName : \?\Volume{772694a4–985d-48ee-9902–104be6c64181}\
BlockRedirectedIOReason : NotBlockRedirected
FileSystemRedirectedIOReason: FileSystemReFs
Name : Cluster Disk 2
Node : savdalhv91
StateInfo : FileSystemRedirected
VolumeFriendlyName : VMs
VolumeName : \?\Volume{772694a4–985d-48ee-9902–104be6c64181}\
In a Hyper-V environment, I recommend making every cluster disk used for storing
virtual machines a CSV. There is no downside, and it enables the efficient mobility of
virtual machines and limits the number of LUNs required.