Non-standard RAID levels
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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.
Now part of RAID 6, double parity, sometimes known as row diagonal parity, like traditional RAID 6, features two sets of parity checks. Differently, the second set is not another set of points in the over-defined 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 consider all A-labeled blocks to produce one or more parity blocks. However, it is fairly easy to calculate parity against multiple groups of blocks, one can 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 double parity RAID 4.
Traditional Twin Syndrome Double parity RAID 4 RAID 4 RAID 4 A1 A2 A3 Ap A1 A2 A3 Ap Aq A1 A2 A3 Ap 1n B1 B2 B3 Bp B1 B2 B3 Bp Bq B1 B2 B3 Bp 2n C1 C2 C3 Cp C1 C2 C3 Cp Cq C1 C2 C3 Cp 3n D1 D2 D3 Dp D1 D2 D3 Dp Dq D1 D2 D3 Dp 4n Note: 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 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 disks through iterative recovery. For example, B2 could be recovered without the use of any x1 or x2 blocks as 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 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 5 or 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.
Intel Matrix RAID
Intel Matrix RAID (a feature of Intel Rapid Storage Technology) is a feature (not a RAID level) present in the ICH6R and subsequent Southbridge chipsets from Intel, accessible via the RAID BIOS. Matrix RAID supports as few as two physical disks or as many as the controller supports. The distinguishing feature of Matrix RAID is that it allows any assortment of RAID 0, 1, 5, and/or 10 volumes in the array, to which a controllable (and identical) portion of each disk is allocated. As such, a Matrix RAID array can improve both performance and data integrity. A practical instance of this would use a small RAID 0 (stripe) for the operating system, program and paging files; and a larger RAID 1 (mirror) to store critical data. Linux MD RAID is also capable of this.
Linux MD RAID 10
The standard "near" layout, where each chunk is repeated n times in a k-way stripe array, is equivalent to the standard RAID 10 arrangement, but it does not require that n evenly divide k. For example an n2 layout on 2, 3 and 4 drives would look like:
2 drives 3 drives 4 drives -------- ---------- -------------- A1 A1 A1 A1 A2 A1 A1 A2 A2 A2 A2 A2 A3 A3 A3 A3 A4 A4 A3 A3 A4 A4 A5 A5 A5 A6 A6 A4 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 software implementation 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 in each section but are switched in groups (for example pairs). 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 A4 A3 A4 A4 A5 A6 A5 A6 A7 A8 A5 A6 A7 A8 A9 A9 A10 A11 A12 .. .. .. .. .. .. .. .. .. A2 A1 A3 A1 A2 A2 A1 A4 A3 A4 A3 A6 A4 A5 A6 A5 A8 A7 A6 A5 A9 A7 A8 A10 A9 A12 A11 .. .. .. .. .. .. .. .. ..
This is designed for striping performance of a mirrored array; sequential reads can be striped, as in RAID-0, random reads 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 performs well for systems where reads are more frequent than writes, which is common. The first 1/f of each drive is a standard 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 A3 A3 A3 A4 A4 A3 A4 A4 A5 A5 A5 A5 A6 A6 A6 A6 A7 A7 A8 A7 A7 A8 A8 A8 A9 A9 A10 A10 .. .. .. .. .. .. .. .. .. A2 A2 A1 A1 A2 A3 A1 A1 A2 A4 A4 A3 A3 A5 A5 A3 A4 A4 A6 A6 A5 A5 A7 A8 A6 A6 A7 A8 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-, and 4-drive arrays are laid out as:
2 drives 3 drives 4 drives -------- ------------ ----------------- A1 A2 A1 A2 A3 A1 A2 A3 A4 A2 A1 A3 A1 A2 A4 A1 A2 A3 A3 A4 A4 A5 A6 A5 A6 A7 A8 A4 A3 A6 A4 A5 A8 A5 A6 A7 A5 A6 A7 A8 A9 A9 A10 A11 A12 A6 A5 A9 A7 A8 A12 A9 A10 A11 .. .. .. .. .. .. .. .. ..
Note: k is the number of drives, n#, f# and o# are parameters in the mdadm
Linux can also create 0, 1, 4, 5, 6 standard RAID configurations using md.
RAID-Z is not actually a kind of RAID, but a higher-level software technology that implements an integrated redundancy scheme in the ZFS file system similar to RAID 5. RAID-Z is a data-protection technology featured by ZFS in order to reduce the block overhead in mirroring.
RAID-Z avoids the RAID 5 "write hole" using copy-on-write; rather than overwriting data, it writes to a new location and then atomically overwrites the pointer to the old data. It avoids 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 file system is aware of the underlying storage structure and can allocate extra space if necessary. RAID-Z2 doubles the parity structure to achieve results similar to RAID 6: the ability to sustain up to two drive failures without losing data. In July 2009, triple-parity RAID-Z3 was added to provide increased redundancy due to the extended resilver times of multi-terabyte disks.
Microsoft announced in 2011 that Drive Extender would no longer be included as part of Windows Home Server Version 2, Windows Home Server 2011 (codename VAIL). As a result there has been a third-party vendor move to fill the void left by DE. Included competitors are Division M, the developers of Drive Bender, DriveHarmony from DataCore, and StableBit's DrivePool.
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Providing RAID over File System (RAID-F) and Transparent RAID (tRAID), FlexRAID is a platform that focuses on flexibility and energy efficiency as well as greater data safety at the lowest risk of data loss. It features N+1 and N+X parity data protection through various RAID engines. All of them are non-standard and have their own nomenclature in the form of Tx, where T stands for Tolerance and x represents the tolerance level. The most acclaimed of them all is the Tx engine providing RAID∞ (infinity) data protection and recovery.
BeyondRAID is not a true RAID extension, but consolidates up to 10 SATA hard drives into one pool of storage. It has the advantage of supporting multiple disk sizes at once, much like JBOD, while providing redundancy for all disks and allowing a hot-swap upgrade at any time. Internally it uses a mix of techniques similar to RAID 1 and RAID 5. Depending on the fraction of data in relation to capacity, it can survive up to three drive failures, if the "array" can be restored onto the remaining good disks before another drive fails. The amount of usable storage can be approximated by summing the capacities of the disks and subtracting the capacity of the largest disk. For example, if a 500, 400, 200, and 100 GB drive were installed, the approximate usable capacity would be 500+400+200+100+(-500)=700 GB of usable space. Internally the data would be distributed in two RAID 5-like arrays and two RAID 1-like sets:
Drives | 100 GB | 200 GB | 400 GB | 500 GB | ---------- | x | unusable space (100 GB) ---------- ------------------- | A1 | A1 | RAID 1 set (2× 100 GB) ------------------- ------------------- | B1 | B1 | RAID 1 set (2× 100 GB) ------------------- ---------------------------- | C1 | C2 | Cp | RAID 5 array (3× 100 GB) ---------------------------- ------------------------------------- | D1 | D2 | D3 | Dp | RAID 5 array (4× 100 GB) -------------------------------------
unRAID is a Linux-based operating system optimized for media file storage.
Disadvantages include slower write performance than a single disk and bottle necking when multiple drives are written concurrently. However, unRAID allows support of a cache drive which dramatically speeds up the write performance. The data is temporarily unprotected until unRAID moves it to the array based on a schedule set within the software. The parity drive must be at least as large as the largest data drive to provide protection.
- Peter Corbett (2004). "Row-Diagonal Parity for Double Disk Failure Correction". USENIX Association. Archived from the original on 2013-11-22. Retrieved 2013-11-22.
- NetApp RAID-DP enables disk firmware updates to occur in real-time without any outage.
- Netapp RAID 4
- http://neil.brown.name/blog/20040827225440 RAID 10 driver
- Main Page - Linux-raid
- Many LSI RAID cards include RAID1E functionality, sometimes called Integrated Mirroring Enhanced.
- user13278091 (31 May 2006). "When to (and Not to) Use RAID-Z". Roch blog. Oracle. Retrieved 29 August 2013.
- Jeff Bonwick (17 November 2005). "RAID-Z". Blog. Oracle. Retrieved 29 August 2013.
- Adam Leventhal's Weblog - Double-Parity RAID-Z
- Adam Leventhal's Weblog - Triple-Parity RAID-Z
- Separate from Windows' Logical Disk Manager
- Data Robotics, Inc. implements BeyondRaid in their Drobostorage device.
- Detailed technical information about BeyondRaid, including how it handles adding and removing drives, is: US US20070266037