Proprietary RAID levels

Although all implementations of RAID differ from the idealized specification to some extent, some companies have developed entirely proprietary RAID implementations that differ substantially from the rest of the crowd.

Double parity

One common addition to the existing RAID levels is double parity, sometimes implemented and known as diagonal parity. As in RAID 6, there are two sets of parity check information created. Unlike RAID 6, the second set is not another set of points in the overdefined polynomial which characterizes the data. Rather, double parity calculates the extra parity against a different group of blocks. For example, in our graph both RAID 5 and RAID 6 calculate against all A-lettered blocks to produce one or more parity blocks. However, as it is fairly easy to calculate parity against multiple groups of blocks, instead of just A-lettered blocks, one can calculate all A-lettered blocks and a permuted group of blocks.
The n blocks are the double parity blocks. The block 2n would be calculated as A2 xor B3 xor Cp, while 3n would be calculated as A3 xor Bp xor C1 and 1n would be calculated as A1 xor B2 xor C3. Because the double parity blocks are correctly distributed it is possible to reconstruct two lost data disks through iterative recovery. For example, B2 could be recovered without the use of any x1 or x2 blocks by computing B3 xor Cp xor 2n = A2, and then A1 can be recovered by A2 xor A3 xor Ap. Finally, B2 = A1 xor C3 xor 1n.

RAID 1.5

RAID 1.5 is a proprietary RAID by HighPoint and is sometimes incorrectly called RAID 15. From the limited information available it appears that it's just a correct implementation of RAID 1. When reading, the data is read from both disks simultaneously and most of the work is done in hardware instead of the driver.

RAID 7

RAID 7 is a trademark of Storage Computer Corporation. It adds caching to RAID 3 or RAID 4 to improve performance.

RAID S or Parity RAID

RAID S is EMC Corporation's proprietary striped parity RAID system used in their Symmetrix storage systems. Each volume exists on a single physical disk, and multiple volumes are arbitrarily combined for parity purposes. EMC originally referred to this capability as RAID S, and then renamed it Parity RAID for the Symmetrix DMX platform. EMC now offers standard striped RAID 5 on the Symmetrix DMX as well.

Matrix RAID

Matrix RAID is a feature that first appeared in the Intel ICH6R RAID BIOS. It is not a new RAID level. Matrix RAID utilizes two physical disks. Part of each disk is assigned to a level 0 array, the other part to a level 1 array. Currently, most (all?) of the other cheap RAID BIOS products only allow one disk to participate in a single array. This product targets home users, providing a safe area (the level 1 section) for documents and other items that one wishes to store redundantly, and a faster area for operating system, applications, etc.

Linux MD RAID 10

The Linux kernel software RAID driver (called md, for "multiple disk") can be used to build a classic RAID 1+0 array, but also has a single level RAID 10 driver with some interesting extensions.
In particular, it supports k-way mirroring on n drives when k does not divide n. This is done by repeating each chunk k times when writing it to an underlying n-way RAID 0 array. For example, 2-way mirroring on 3 drives would look like

IBM ServeRAID 1E

The IBM ServeRAID adapter series supports 2-way mirroring on an arbitrary number of drives.
This configuration is tolerant of non-adjacent drives failing. Other storage systems including Sun's StorEdge T3 support this mode as well.

RAID Z

Sun's ZFS implements an integrated redundancy scheme similar to RAID 5 which it calls RAID Z. RAID Z avoids the RAID 5 "write hole" and the need for read-modify-write operations for small writes by only ever performing full-stripe writes; small blocks are mirrored instead of parity protected, which is possible because the filesystem is aware of the underlying storage structure and can allocate extra space if necessary.

Jagath Krishnakumar