Windows Storage Space allocation unit size?

Patrick3D · July 22, 2016

I hit the 16TB limit of the default 4Kb allocation unit size with my Windows Storage Spaces last night. The problem is, I cannot find any way in the Storage Spaces Management GUI to specify an allocation unit size. I recreated the storage spaces using the automated wizard and they are now using 8Kb but I would like to manually set them to 16Kb to avoid running into this issue in the near future. I've searched but cannot seem to find a clear tutorial for doing so. I assume this is going to require some usage of PowerShell? Can it be done through the Disk Management utility instead?

leadeater · July 22, 2016

The limit you have hit is due to partitioning and not Storage Spaces. Storage Spaces LogicalSectorSize is what you set on the entire storage pool which defines the size of sectors to write down to each physical disk in the pool. You need to set this value carefully, if you have SSDs in your pool you need to manually configure it to 512 bytes. If you have large NAS rated disks they will have a physical sector size of 4k and Windows by default when creating a storage pool will use the largest detected physical sector size.

However SSDs report a physical sector size of 512 bytes but in reality they are 4k, this is known as 512e. What makes this so bad if your pool is set to 4k is that for every write to the SSD the OS will perform an RMW action to split the 4k in to 512 then write down to the SSD then the SSD will also perform an RMW action to combine each 512 in to 4k, this absolutely kills the SSD performance.

http://jeffgraves.me/2014/06/03/ssds-on-storage-spaces-are-killing-your-vms-performance/

For your actual problem you will need to backup all your data, or create a new virtual disk, and re-partition your volume using a larger Allocation Unit Size.

Edit: You also can not change the Storage Space LogicalSectorSize after creation, this a one time thing and is either 512b or 4k. Choose wisely or much pain can come later, I had to backup everything and re-create everything which was a real pain.