Definition - What does RAID 51 mean?
RAID 51 is a type of nested RAID level that provides RAID 1-based mirroring on each RAID 5 array.
It is similar to RAID 1, where each element is a disk with native redundancy capabilities. It requires a minimum of six disks to operate.
RAID 51 is also known as RAID 5+1.
Techopedia explains RAID 51
RAID 51 is implemented by mirroring or implementing RAID 1 on an entire RAID 5 array in addition to the parity information. It is generally created using software and hardware-based RAID techniques where RAID 1-based mirroring is implemented through an operating system on the hardware-based RAID 5 array.
RAID 51 is specifically designed for enhanced backup availability and high fault tolerance capabilities. RAID 51 is considered a parity set of mirrored disks, hence RAID 5 is followed by RAID 1. It can remain operational or protect from data loss even after losing four of the six minimum configured disks.