Windows Server 2012 introduced Storage Spaces as a method of having virtualized storage. In this initial release there were three choices for storage layout
One of the improvements in Windows Server 2012 R2, is to the parity space. Originally, parity was pretty much a RAID5, allowing for the loss of a single disk. In the new release, we will be able to create ‘dual parity’. This new option will allow you to have a storage space that can survive the loss of any two disks.
A dual parity layout requires at least seven disks. If less than seven disks are present in the pool, the dual parity option will not be visible.