Nested RAID levels

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. At least four disks are required in a standard RAID 01 configuration, but larger arrays are also used. == (RAID 0+3)==

RAID 03, also called RAID 0+3 and sometimes RAID 53, is similar to RAID 01 with the exception that byte-level striping with dedicated parity is used instead of mirroring. == (RAID 1+0) ==

RAID 10, also called RAID 1+0 and sometimes RAID 1&0, is similar to RAID 01 with an exception that the two used standard RAID levels are layered in the opposite order; thus, RAID 10 is a stripe of mirrors. and requires a minimum of four drives. However, a nonstandard definition of "RAID 10" was created for the Linux MD driver; Linux "RAID 10" can be implemented with as few as two disks. Implementations supporting two disks, such as Linux RAID 10, offer a choice of layouts. Arrays of more than four disks are also possible. According to manufacturer specifications and official independent benchmarks, in most cases RAID 10 provides better throughput and latency than all other RAID levels except RAID 0 (which wins in throughput). Thus, it is the preferable RAID level for I/O-intensive applications such as database, email, and web servers, as well as for any other use requiring high disk performance. == (RAID 5+0) ==

RAID 50, also called RAID 5+0, combines the straight block-level striping of RAID 0 with the distributed parity of RAID 5. RAID 50 improves upon the performance of RAID 5 particularly during writes, and provides better fault tolerance than a single RAID level does. This level is recommended for applications that require high fault tolerance, capacity and random access performance. As the number of drives in a RAID set increases, and the capacity of the drives increase, this impacts the fault-recovery time correspondingly as the interval for rebuilding the RAID set increases. == (RAID 6+0) ==

RAID 60, also called RAID 6+0, combines the straight block-level striping of RAID 0 with the distributed double parity of RAID 6, resulting in a RAID 0 array striped across RAID 6 elements. It requires at least eight disks. == (RAID 10+0) ==

RAID 100, sometimes also called RAID 10+0, is a stripe of RAID 10s. This is logically equivalent to a wider RAID 10 array, but is generally implemented using software RAID 0 over hardware RAID 10. Being "striped two ways", RAID 100 is described as a "plaid RAID". == Comparison ==

The following table provides an overview of some considerations for nested RAID levels. In each case: • Space efficiency is given as an expression in terms of the number of drives, ; this expression designates a fractional value between zero and one, representing the fraction of the sum of the drives' capacities that is available for use. For example, if three drives are arranged in RAID 3, this gives an array space efficiency of ; thus, if each drive in this example has a capacity of 250 GB, then the array has a total capacity of 750 GB but the capacity that is usable for data storage is only 500 GB. It is sometimes necessary to use m in place of n due to the inherent nature of the configuration (of use in RAID 10). Fault tolerance uses m for representation, in place of n, in certain Nested RAID levels (see below for fault tolerance calculation). m is the number of disks in each mirror, rather than the total number of disks. • Fault tolerance is the number of drive failures allowed, where min is the guaranteed number of failures the RAID can handle and max is the maximum possible without guaranteed failure. == See also ==