RAID 01, also known as RAID 0+1, is a type of nested RAID level that uses a mirror of stripes, achieving both replication and sharing of data between disks. The usable capacity of a RAID 01 disk array is the same as in RAID 1 array made of the same drives, wherein half of the disk drive capacity is utilized mirroring the other half.
Primarily, RAID 01 mirrors data of the stripes. It stripes several disks together into sets which are mirrored together. RAID 01 offers good fault-tolerance capabilities wherein operation is not interrupted with the failure of a single disk drive. But if one drive from each disk is lost, the RAID 01 function is affected.
A standard RAID 01 configuration requires a minimum of four disks arrays; however, larger disks arrays can also be used.