RAID 51, also known as RAID 5+1, is a nested redundant array of independent disks array that provides RAID 1-based mirroring on each RAID 5 array. Similar to RAID 1, its each element is a disk with native redundancy capabilities.
RAID 51 requires at least six disks to constitute.
Usually, RAID 51 is implemented by mirroring or implementing RAID 1 on a whole RAID 5 array including the parity information. Generally, RAID 51 array is constituted by the use of hardware and software based RAID technologies, wherein RAID 1-based mirroring is applied through an operating system on the hardware-based RAID 5 array. It is especially constructed for increased backup availability and high fault-tolerance capabilities.
This [RAID] is treated as a parity set of mirrored disks, hence RAID 5 is followed by RAID 1. The biggest advantage of using RAID 51 array is that it can remain operational or protect from losing data even after failure of four of the six minimum configured disks.