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Non-standard RAID levels 1 Non-standard RAID levels Although all RAID implementations differ from the specification to some extent, some companies have developed non-standard RAID implementations that differ substantially from the standard. Non-RAID drive architectures—configurations of multiple hard drives are not referred to by RAID acronyms. Double parity Now part of RAID 6, double parity, sometimes known as row diagonal parity,[1] like traditional RAID 6, features two sets of parity check
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  Non-standard RAID levels1 Non-standard RAID levels Although all RAID implementations differ from the specification to some extent, some companies have developed non-standard RAID implementations that differ substantially from the standard. Non-RAID drivearchitectures  — configurations of multiple hard drives are not referred to by RAID acronyms. Double parity Diagram of a RAID DP (Double Parity) setup. Now part of RAID 6, double parity , sometimes known as rowdiagonal parity , [1] like traditional RAID 6, features two sets of paritychecks. Differently, the second set is not another set of points in theover-defined polynomial which characterizes the data. Rather, doubleparity calculates the extra parity against a different group of blocks. [2] For example, in our graph both RAID 5 and RAID 6 consider allA-labeled blocks to produce one or more parity blocks. However, it isfairly easy to calculate parity against multiple groups of blocks, onecan calculate all A blocks and a permuted group of blocks.This is more easily illustrated using RAID 4, Twin Syndrome RAID 4 (RAID 6 with a RAID 4 layout), and doubleparity RAID 4. Traditional Twin Syndrome Double parityRAID 4 RAID 4 RAID 4A1 A2 A3 Ap A1 A2 A3 Ap Aq A1 A2 A3 Ap 1nB1 B2 B3 Bp B1 B2 B3 Bp Bq B1 B2 B3 Bp 2nC1 C2 C3 Cp C1 C2 C3 Cp Cq C1 C2 C3 Cp 3nD1 D2 D3 Dp D1 D2 D3 Dp Dq D1 D2 D3 Dp 4nNote: A1, B1, et cetera each represent one data block; each column represents one disk. The n blocks are the double parity blocks. Block 2n is A2 xor B3 xor Cp, while 3n is A3 xor Bp xor C1 and 1nwould be calculated as A1 xor B2 xor C3. Because the double parity blocks are correctly distributed it is possible toreconstruct two lost disks through iterative recovery. For example, B2 could be recovered without the use of any x1or x2 blocks as B3 xor Cp xor 2n = A2, and then A1 can be recovered by A2 xor A3 xor Ap. Finally, B2 = A1 xorC3 xor 1n.Running in degraded mode with double parity increases risk of data loss. RAID-DP RAID-DP [3] implements double parity within RAID 6. [4] The performance penalty of RAID-DP is typically under2% vs a comparable RAID 4. [5] File system requests are first written to the battery  – backed NVRAM to prevent dataloss should the system lose power. Blocks are not updated in place. Writes are aggregated and the storage controllertries to write only complete stripes including both parity blocks. RAID-DP provides better protection and equal orbetter performance than RAID 10, and in most cases doesn't suffer from traditional RAID 6 challenges of in-placeblock updating and spreads reads and writes over more disks when compared to a RAID 6 group of same size.  Non-standard RAID levels2 RAID 1.5 RAID 1.5 is a proprietary [6] RAID and is sometimes incorrectly called RAID 15. RAID 1.5 performs data stripingand mirroring using two hard drives. Data is read from both disks simultaneously and most of the work is done inhardware instead of the driver.Linux's and Solaris's RAID 1 implementations read from both disks simultaneously, so RAID 1.5 offers no otherbenefit. RAID 5E, RAID 5EE and RAID 6E RAID 5E, RAID 5EE and RAID 6E (with the added  E  standing for  Enhanced  ) generally refer to variants of RAID 5or RAID 6 with an integrated hot-spare drive, where the spare drive is an active part of the block rotation scheme.This spreads I/O across all drives, including the spare, thus reducing the load on each drive, increasing performance.It does, however, prevent sharing the spare drive among multiple arrays, which is occasionally desirable. [7] These systems do not dedicate one drive as the spare drive , just as RAID 5 or RAID 6 do not dedicate one drive asthe parity drive . Instead, spare blocks are distributed across all drives, so that in a 10-disk RAID 5E with one spare , every disk is 80% data, 10% parity, and 10% spare. The spare blocks in RAID 5E and RAID 6E are at theend of the array, while in RAID 5EE the spare blocks are integrated into the array. RAID 5EE can sustain a singledrive failure. RAID 5EE requires at least four disks and can expand to 16 disks. A drive failure in a RAID 5E/5EEarray compresses the array and re-stripes it into a standard RAID 5 array. This process can be very I/O intensive andmay take hours or days depending on the speed, size and number of drives. Only after compression can the systemprevent data loss given a second drive failure. Replacing the failed drive after compression permits restructuring intoa RAID 5E/5EE array. The length of time and intense I/O of both compression and decompression creates a practicallimit of 4 to 8 drives. The performance boost diminishes after 8 drives. Parity RAID Parity RAID is EMC Corporation's proprietary striped parity RAID system used in their Symmetrix storagesystems. The system places each volume on a single physical disk, and arbitrarily combines multiple volumes forparity purposes. EMC srcinally referred to this capability as RAID S , and then renamed it Parity RAID for theSymmetrix DMX platform. EMC now offers standard striped RAID 5 on the Symmetrix DMX as well. Traditional EMCRAID 5 RAID SA1 A2 A3 Ap A1 B1 C1 1pB1 B2 Bp B3 A2 B2 C2 2pC1 Cp C2 C3 A3 B3 C3 3pDp D1 D2 D3 A4 B4 C4 4p Note: A1, B1, et cetera each represent one data block; each column represents one disk.A, B, et cetera are entire volumes.  Non-standard RAID levels3 Intel Rapid Storage Technology Diagram of a Matrix RAID setup. Intel Rapid Storage Technology (formerly called Intel Matrix RAID) isa feature (not a RAID level) present in the ICH6R and subsequentSouthbridge chipsets from Intel, accessible via the RAID BIOS. MatrixRAID supports as few as two physical disks or as many as thecontroller supports. The distinguishing feature of Matrix RAID is thatit allows any assortment of RAID 0, 1, 5, and/or 10 volumes in thearray, to which a controllable (and identical) portion of each disk isallocated. As such, a Matrix RAID array can improve bothperformance and data integrity. A practical instance of this would use asmall RAID 0 (stripe) for the operating system, program and pagingfiles; and a larger RAID 1 (mirror) to store critical data. Linux MDRAID is also capable of this. Linux MD RAID 10 The Linux kernel software RAID driver (called md, for multipledevice ) can be used to build a classic RAID 1+0 array, but also as a single level [8] with some interestingextensions. [9] The standard near layout, where each chunk is repeated n times in a k  -way stripe array, is equivalent to thestandard RAID 10 arrangement, but it does not require that n evenly divide k  . For example an n2 layout on 2, 3 and 4drives would look like: 2 drives 3 drives 4 drives-------- ---------- --------------A1 A1 A1 A1 A2 A1 A1 A2 A2A2 A2 A2 A3 A3 A3 A3 A4 A4A3 A3 A4 A4 A5 A5 A5 A6 A6A4 A4 A5 A6 A6 A7 A7 A8 A8.. .. .. .. .. .. .. .. .. The 4-drive example is identical to a standard RAID-1+0 array, while the 3-drive example is a softwareimplementation of RAID-1E. The 2-drive example is equivalent to RAID 1.The driver also supports a far layout where all the drives are divided into  f  sections. All the chunks are repeated ineach section but offset by one device. For example,  f2 layouts on 2-, 3-, and 4-drive arrays would look like: 2 drives 3 drives 4 drives-------- -------------- --------------------A1 A2 A1 A2 A3 A1 A2 A3 A4A3 A4 A4 A5 A6 A5 A6 A7 A8A5 A6 A7 A8 A9 A9 A10 A11 A12.. .. .. .. .. .. .. .. ..A2 A1 A3 A1 A2 A4 A1 A2 A3A4 A3 A6 A4 A5 A8 A5 A6 A7A6 A5 A9 A7 A8 A12 A9 A10 A11.. .. .. .. .. .. .. .. ..  Non-standard RAID levels4This is designed for striping performance of a mirrored array; sequential reads can be striped, as in RAID-0, randomreads are somewhat faster (maybe 10-20 % due to using the faster outer disk sectors, and smaller average seek times), and sequential and random writes offer about equal performance to other mirrored raids. The layout performswell for systems where reads are more frequent than writes, which is common. The first 1/   f  of each drive is astandard RAID-0 array. This offers striping performance on a mirrored set of only 2 drives.The near and far options can be used together. The chunks in each section are offset by n device(s). For example n2 f2 layout stores 2×2 = 4 copies of each sector, so requires at least 4 drives: A1 A1 A2 A2 A1 A1 A2 A2 A3A3 A3 A4 A4 A3 A4 A4 A5 A5A5 A5 A6 A6 A6 A6 A7 A7 A8A7 A7 A8 A8 A8 A9 A9 A10 A10.. .. .. .. .. .. .. .. ..A2 A2 A1 A1 A2 A3 A1 A1 A2A4 A4 A3 A3 A5 A5 A3 A4 A4A6 A6 A5 A5 A7 A8 A6 A6 A7A8 A8 A7 A7 A10 A10 A8 A9 A9.. .. .. .. .. .. .. .. .. The driver also supports an offset layout where each stripe is repeated o times. For example, o2 layouts on 2-, 3-, and4-drive arrays are laid out as: 2 drives 3 drives 4 drives-------- ------------ -----------------A1 A2 A1 A2 A3 A1 A2 A3 A4A2 A1 A3 A1 A2 A4 A1 A2 A3A3 A4 A4 A5 A6 A5 A6 A7 A8A4 A3 A6 A4 A5 A8 A5 A6 A7A5 A6 A7 A8 A9 A9 A10 A11 A12A6 A5 A9 A7 A8 A12 A9 A10 A11.. .. .. .. .. .. .. .. .. Note: k  is the number of drives, n#  ,  f#  and o#  are parameters in the mdadm --layout option.Linux can also create 0, 1, 4, 5, 6 standard RAID configurations using md.
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