Why is Storage Spaces virtual disk efficiency with parity not consistent with the number disks allocated?

Question

I have a 4x8TB (29TiB total) hard drive array in a storage pool on a Windows 10 PC. I am attempting to create a 24TB (21.75 TiB) array with 1 disk's worth of redundancy. I execute the following PS command:

PS C:\Windows\system32> New-VirtualDisk -StoragePoolFriendlyName "32TB Array"  -FriendlyName "24TB Filesystem" -ProvisioningType Fixed -ResiliencySettingName "Parity" -PhysicalDiskRedundancy 1 -AutoNumberOfColumns  -UseMaximumSize -WriteCacheSize 1GB

And it gives me this result:

Why is it giving me a storage efficiency of only 66.66%? Any way to fix this so it's 75% (which is what I'd expect in a 4 drive RAID 5 array)?

FWIW, I tried the same thing in Windows 11 and got the same results.

Answer 1

Hi Forlsackth,

Hope you're doing well.

The storage efficiency of 66.66% indicates that the actual usable capacity of the virtual disk is 66.66% of the total physical capacity. This is due to the overhead required for parity data in a parity-based resiliency setting like RAID 5.

In a 4-drive RAID 5 array, one drive's worth of capacity is used for parity data, leaving 3 drives for actual data storage. Therefore, the usable capacity is 3/4 or 75% of the total physical capacity.

However, in your case, you specified "PhysicalDiskRedundancy 1", which means you want one disk's worth of redundancy. In a parity-based resiliency setting like RAID 5, this means that the capacity of one entire disk is reserved for parity data to provide redundancy against the failure of any single disk.

To achieve a storage efficiency of 75%, you would need to specify "PhysicalDiskRedundancy 0" in your command, indicating that you want no additional disk redundancy beyond the parity data. This assumes that you are comfortable with the risk of data loss in case one of the disks fails.

Best Regards,

Ian Xue

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Answer 2

If that were the case, then why wouldn't the available capacity be 50% rather than 66%? And when I tried creating the array using the Storage Spaces GUI (there are no options there to choose drive redundancy), I got the same results.