Nested (hybrid) RAID
In what was originally termed hybrid RAID, many storage controllers allow RAID levels to be nested. The elements of a RAID may be either individual drives or RAIDs themselves. However, if a RAID is itself an element of a larger RAID, it is unusual for its elements to be themselves RAIDs.
As there is no basic RAID level numbered larger than 9, nested RAIDs are usually clearly described by attaching the numbers indicating the RAID levels, sometimes with a "+" in between. The order of the digits in a nested RAID designation is the order in which the nested array is built: For a RAID 1+0, drives are first combined into multiple level 1 RAIDs that are themselves treated as single drives to be combined into a single RAID 0; the reverse structure is also possible (RAID 0+1).
The final RAID is known as the top array. When the top array is a RAID 0 (such as in RAID 1+0 and RAID 5+0), most vendors omit the "+" (yielding RAID 10 and RAID 50, respectively).
- RAID 0+1: striped sets in a mirrored set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.
- The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost.
- RAID 1+0: (a.k.a. RAID 10) mirrored sets in a striped set (minimum four drives; even number of drives) provides fault tolerance and improved performance but increases complexity.
- The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses so long as no mirror loses all its drives.
- RAID 5+3: mirrored striped set with distributed parity (some manufacturers label this as RAID 53).
Whether an array runs as RAID 0+1 or RAID 1+0 in practice is often determined by the evolution of the storage system. A RAID controller might support upgrading a RAID 1 array to a RAID 1+0 array on the fly, but require a lengthy off-line rebuild to upgrade from RAID 1 to RAID 0+1. With nested arrays, sometimes the path of least disruption prevails over achieving the preferred configuration.
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