Nested RAID levels
stacked combination of two or more standard RAID levels
Nested RAID levels, also known as hybrid RAID, combine two or more of the standard RAID levels (where "RAID" stands for "redundant array of independent disks" or "redundant array of inexpensive disks") to gain performance, additional redundancy or both, as a result of combining properties of different standard RAID layouts.
Nested RAID levels are usually numbered using a series of numbers, where the most commonly used levels use two numbers. The first number in the numeric designation denotes the lowest RAID level in the "stack", while the rightmost one denotes the highest layered RAID level; for example, RAID 50 layers the data striping of RAID 0 on top of the distributed parity of RAID 5. Nested RAID levels include RAID 01, RAID 10, RAID 100, RAID 50 and RAID 60, which all combine data striping with other RAID techniques; as a result of the layering scheme, RAID 01 and RAID 10 represent significantly different nested RAID levels.
RAID 01 (RAID 0+1)
RAID 01, also called RAID 0+1, is a RAID level using a mirror of stripes, achieving both replication and sharing of data between the disks. The usable capacity of a RAID 01 array is the same as in a RAID 1 array made of the same drives, in which one half of the drives is used to mirror the other half.
(
N
/
2
)
⋅
S
m
i
n
{\displaystyle (N/2)\cdot S_{\mathrm {min} }}
, where
N
{\displaystyle N}
is the total number of drives and
S
m
i
n
{\displaystyle S_{\mathrm {min} }}
is the capacity of the smallest drive in the array.
At least four disks are required in a standard RAID 01 configuration, but larger arrays are also used.
If a single drive failure occurs then an entire stripe set has failed, at this point it is running effectively as RAID 0 with no redundancy. The probability of failure of a RAID 0 stripe set is always higher than that of a single disk, and increases with every disk added to it.
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