Redundant array of independent disks |
In Computing, a redundant array of independent disks (more commonly known as a RAID) is a system of using multiple hard drives for sharing or replicating Data among the drives. Depending on the version chosen, the benefit of RAID is one or more of increased data integrity, fault-tolerance, throughput or capacity compared to single drives. In its original implementations (in which it was an abbreviation for Redundant Array of Inexpensive Disks ), its key advantage was the ability to combine multiple low-cost devices using older technology into an array that offered greater capacity, reliability, or speed, or a combination of these things, than was affordably available in a single device using the newest technology.
At the very simplest level, RAID combines multiple hard drives into one single logical unit. Thus, instead of seeing several different hard drives, the operating system sees only one. RAID is typically used on server computers, and is usually implemented with identically-sized disk drives. With decreases in hard drive prices and wider availability of RAID options built into motherboard chipsets, RAID is also being found and offered as an option in more advanced end user computers. This is especially true in computers dedicated to storage-intensive tasks, such as video and audio editing.
The original RAID specification suggested a number of prototype RAID levels , or combinations of disks. Each had theoretical advantages and disadvantages. Over the years, different implementations of the RAID concept have appeared. Most differ substantially from the original idealized RAID levels, but the numbered names have remained. This can be confusing, since one implementation of RAID 5, for example, can differ substantially from another. RAID 3 and RAID 4 are often confused and even used interchangeably.
The very definition of RAID has been argued over the years. The use of the term redundant leads many to split hairs over whether RAID 0 is a real RAID type. Similarly, the change from inexpensive to independent confuses many as to the intended purpose of RAID. There are even some single-disk implementations of the RAID concept. For the purpose of this article, we will say that any system which employs the basic RAID concepts to recombine physical disk space for purposes of reliability, capacity, or performance is a RAID system.
= History =
RAID was first patented by Conference 1988: pp 109–116. The term RAID started with this paper.
It was particularly ground-breaking work in that the concepts are both novel and obvious in retrospect once they have been described. This paper spawned the entire disk array industry.
= RAID implementations =
== Inexpensive vs. independent ==
While the I in RAID now generally means independent rather than inexpensive , one of the original benefits of RAID was that it did use inexpensive equipment, and this still holds true in many situations, where Advanced Technology Attachment disks are used.
More commonly, independent (and more expensive) SCSI hard disks are used, although the cost of such disks is now much lower than it once was and much lower than the systems RAID was originally intended to replace.
== Hardware vs. software ==
RAID can be implemented either in dedicated hardware or custom Software running on standard hardware. Additionally, there are hybrid RAIDs that are partly software- and partly hardware-based solutions.
With a software implementation, the operating system manages the disks of the array through the normal drive controller (IDE, SCSI, Fibre Channel or any other). With present CPU speeds, software RAID can be faster than hardware RAID, though at the cost of using CPU power which might be best used for other tasks. One major exception is where the hardware implementation of RAID incorporates a battery backed-up write cache and an application, like a database server. In this case, the hardware RAID implementation flushes the write cache to a secure storage to preserve data at a known point if there is a crash. The hardware approach is faster and limited instead by RAM speeds, the amount of cache and how fast it can flush the cache to disk. For this reason, battery-backed caching disk controllers are often recommended for high transaction rate database servers. In the same situation, the software solution is limited to no more flushes than the number of rotations or seeks per second of the drives. Another disadvantage of a pure software RAID is that, depending on the disk that fails and the boot arrangements in use, the computer may not be able to be rebooted until the array has been rebuilt.
A hardware implementation of RAID requires (at a minimum) a special-purpose RAID controller. On a desktop system, this may be a PCI expansion card, or might be a capability built in to the motherboard. In larger RAIDs, the controller and disks are usually housed in an external multi-bay enclosure. The disks may be IDE, Advanced Technology Attachment, Serial ATA, SCSI, Fibre Channel, or any combination thereof. The controller links to the host computer(s) with one or more high-speed SCSI, Fibre Channel or Internet SCSI connections, either directly, or through a fabric, or is accessed as network attached storage. This controller handles the management of the disks, and performs parity calculations (needed for many RAID levels). This option tends to provide better performance, and makes operating system support easier. Hardware implementations also typically support hot swapping, allowing failed drives to be replaced while the system is running. In rare cases hardware controllers have become faulty, which can result in data loss. Because of this drawback, software RAID is a slightly more reliable and safer option.
Hybrid RAIDs have become very popular with the introduction of very cheap hardware RAID controllers . The hardware is just a normal disk controller that has no RAID features, but there is a boot-time set-up application that allows users to set up RAIDs that are controlled via the BIOS. When any modern operating systems are used, they will need specialized RAID drivers that will make the array look like a single block device. Since these controllers actually do all the calculations in software, not hardware, they are often called fakeraids . Unlike software RAID, these fakeraids typically cannot span multiple controllers.
Both hardware and software versions may support the use of a hot spare , a preinstalled drive which is used to immediately (and almost always automatically) replace a failed drive. This cuts down the time period in which a second failure can take out the array.
Some software RAID systems allow building arrays from partitions instead of whole disks. Unlike #Matrix_RAID they are not limited to just RAID 0 and RAID 1 and not all partitions have to be RAID.
= Standard RAID levels =
== RAID 0 ==
A RAID 0 (also known as a striped set) splits data evenly across two or more disks with no parity information for redundancy. It is important to note that RAID 0 was not one of the original RAID levels, and is not redundant. RAID 0 is normally used to increase performance, although it can also be used as a way to create a small number of large virtual disks out of a large number of small physical ones. A RAID 0 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk—for example, if a 120 GB disk is striped together with a 100 GB disk, the size of the array will be 200 GB. Although RAID 0 was not specified in the original RAID paper, an idealized implementation of RAID 0 would split I/O operations into equal-sized blocks and spread them evenly across two disks. RAID 0 implementations with more than two disks are also possible, however the reliability of a given RAID 0 set is equal to the average reliability of each disk divided by the number of disks in the set. That is, reliability (as measured by Failure rate (MTTF) or Failure rate (MTBF)) is roughly inversely proportional to the number of members—so a set of two disks is roughly half as reliable as a single disk. The reason for this is that the file system is distributed across all disks. When a drive fails the file system cannot cope with such a large loss of data and coherency since the data is striped across all drives. Data can be recovered using special tools. However, it will be incomplete and most likely corrupt.
While the block size can technically be as small as a byte it is almost always a multiple of the hard disk sector size of 512 bytes. This lets each drive seek independently when randomly reading or writing data on the disk. If all the accessed sectors are entirely on one disk then the apparent seek time would be the same as a single disk. If the accessed sectors are spread evenly among the disks then the apparent seek time would be reduced by half for two disks, by two-thirds for three disks, etc. assuming identical disks. For normal data access patterns the apparent seek time of the array would be between these two extremes. The transfer speed of the array will be the transfer speed of all the disks added together.
Traditional RAID 0 A1 A2 A3 A4 A5 A6 A7 A8
Note: A1, B1, et cetera each represent one data block.
RAID 0 is useful for set-ups such as large read-only Network_File_System servers where mounting many disks is time-consuming or impossible and redundancy is irrelevant. Another use is where the number of disks is limited by the operating system. In Microsoft Windows, the number of drive letters for hard disk drives may be limited to 24, so RAID 0 is a popular way to use more than this many disks. It is also a popular choice for gaming systems where performance is desired. However, since data is shared between drives without redundancy, hard drives cannot be swapped out as all disks are dependent upon each other.
Also with RAID 0 if one drive failed you may have lost a lot more than if you had your data spread across separate drives as files with bits missing can be of very limited usefulness and you cannot copy important data manually to multiple drives (unless you have multiple separate arrays).
== Concatenation (JBOD) ==
Although a concatenation of disks (also called JBOD, or Just a Bunch of Disks ) is not one of the numbered RAID levels, it is a popular method for combining multiple physical disk drives into a single virtual one. As the name implies, disks are merely Concatenate together, end to beginning, so they appear to be a single large disk.
In this sense, concatenation is akin to the reverse of drive partition. Whereas partitioning takes one physical drive and creates two or more logical drives, JBOD uses two or more physical drives to create one logical drive.
In that it consists of an Array of Inexpensive Disks (no redundancy), it can be thought of as a distant relation to RAID. JBOD is sometimes used to turn several odd-sized drives into one useful drive. Therefore, JBOD could use a 3 GB, 15 GB, 5.5 GB, and 12 GB drive to combine into a logical drive at 35.5 GB, which is often more useful than the individual drives separately.
One advantage JBOD has over RAID 0 is in the case of drive failure. Whereas in RAID 0, failure of a single drive will usually result in the loss of all data in the array, in a JBOD array only the data on the affected drive is lost, and the data on surviving drives will remain readable.
== RAID 1 ==
A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful for set-ups where redundancy is more important than using all the disks maximum storage capacity. The array can only be as big as the smallest member disk, however. An ideal RAID 1 set contains two disks, which increases reliability by a factor of two over a single disk, but it is possible to have many more than two copies. Since each member can be addressed independently if the other fails, reliability is a linear multiple of the number of members. To truly get the full redundancy benefits of RAID 1, independent disk controllers are recommended, one for each disk. Some refer to this practice as splitting or duplexing.
When reading both disks can be accessed independently. Like RAID 0 the average seek time is reduced by half when randomly reading but because each disk has the exact same data the requested sectors can always be split evenly between the disks and the seek time remains low. The transfer rate would also be doubled. For three disks the seek time would be a third and the transfer rate would be tripled. The only limit is how many disks can be connected to the controller and its maximum transfer speed. Most IDE RAID 1 cards use a broken implementation and only read from one disk so their read performance is that of a single disk. Some older RAID 1 implementations would read both disks simultaneously and compare the data to catch errors. The error detection and correction on modern disks makes this no longer necessary. When writing, the array acts like a single disk as all writes must be written to all disks.
RAID1 has many administrative advantages. For instance, in some 365*24 environments, it is possible to Split the Mirror : declare one disk as inactive, do a backup of that disk, and then rebuild the mirror. This procedure is less critical in the presence of the snapshot feature of some filesystems, in which some space is reserved for changes, presenting a static point-in-time view of the filesystem. Alternatively, a set of disks can be kept in much the same way as traditional backup tapes are.
Also, one common practice is to create an extra mirror of a volume (also known as a Business Continuance Volume or BCV) which is meant to be split from the source RAID set and used independently. In some implementations, these extra mirrors can be split and then incrementally re-established, instead of requiring a complete RAID set rebuild.
Traditional RAID 1 A1 A1 A2 A2 A3 A3 A4 A4
Note: A1, B1, et cetera each represent one data block.
== RAID 2 ==
A RAID 2 stripes data at the Bit (rather than block) level, and uses a Hamming code for error correction. The disks are synchronized by the controller to run in perfect tandem. This is the only original level of RAID that is not currently used.
== RAID 3 ==
A RAID 3 uses byte-level striping with a dedicated parity disk. RAID 3 is very rare in practice. One of the side effects of RAID 3 is that it generally cannot service multiple requests simultaneously. This comes about because any single block of data will by definition be spread across all members of the set and will reside in the same location, so any I/O operation requires activity on every disk.
In our example below, a request for block A1 would require all three data disks to seek to the beginning and reply with their contents. A simultaneous request for block B1 would have to wait.
Traditional RAID 3 A1 A2 A3 Ap(1-3) A4 A5 A6 Ap(4-6) A7 A8 A9 Ap(7-9) B1 B2 B3 Bp(1-3)
Note: A1, B1, et cetera each represent one data byte.
== RAID 4 ==
A RAID 4 uses block-level striping with a dedicated parity disk. RAID 4 looks similar to RAID 3 except that it stripes at the block, rather than the byte level. This allows each member of the set to act independently when only a single block is requested. If the disk controller allows it, a RAID 4 set can service multiple read requests simultaneously. Network Appliance uses RAID 4 on their Filer line of network storage servers.
In our example below, a request for block A1 would be serviced by disk 1. A simultaneous request for block B1 would have to wait, but a request for B2 could be serviced concurrently.
Traditional RAID 4 A1 A2 A3 Ap B1 B2 B3 Bp C1 C2 C3 Cp D1 D2 D3 Dp
Note: A1, B1, et cetera each represent one data block.
== RAID 5 ==
A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 is one of the most popular RAID levels, and is frequently used in both hardware and software implementations. Virtually all storage arrays offer RAID 5. As with RAID 0, RAID 5 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk—for example, if a 120 GB disk is used to build a RAID 5 together with two 100 GB disks, each disk will donate 100 GB to the array for a total of 200 GB of storage. 100 GB are used for parity information, and the excess 20 GB from the larger disk are ignored.
In our example below, a request for block A1 would be serviced by disk 1. A simultaneous request for block B1 would have to wait, but a request for B2 could be serviced concurrently.
Traditional RAID 5 A1 A2 A3 Ap B1 B2 Bp B3 C1 Cp C2 C3 Dp D1 D2 D3
Note: A1, B1, et cetera each represent one data block.
Every time a data block (sometimes called a chunk ) is written on a disk in an array, a parity block is generated within the same stripe. (A block or chunk is often composed of many consecutive sectors on a disk, sometimes as many as 256 sectors. A series of chunks [a chunk from each of the disks in an array] is collectively called a stripe .) If another block, or some portion of a block is written on that same stripe, the parity block (or some portion of the parity block) is recalculated and rewritten. The disk used for the parity block is staggered from one stripe to the next, hence the term distributed parity blocks . This means, of course, that the controller software becomes more complex.
Interestingly, the parity blocks are not read on data reads, since this would be unnecessary overhead and would diminish performance. The parity blocks are read, however, when a read of a data sector results in a cyclic redundancy check (CRC) error. In this case, the sector in the same relative position within each of the remaining data blocks in the stripe and within the parity block in the stripe are used to reconstruct the errant sector. The CRC error is thus hidden from the main computer. Likewise, should a disk fail in the array, the parity blocks from the surviving disks are combined mathematically with the data blocks from the surviving disks to reconstruct the data on the failed drive on the fly .
This is sometimes called Interim Data Recovery Mode. The computer knows that a disk drive has failed, but this is only so that the operating system can notify the administrator that a drive needs replacement; applications running on the computer are unaware of the failure. Reading and writing to the drive array continues seamlessly, though with some performance degradation. The difference between RAID 4 and RAID 5 is that, in Interim data recovery mode, RAID 5 might be slightly faster than RAID 4, because, when the CRC and parity are in the disk that failed, the calculation does not have to be performed, while with RAID 4, if one of the data disks fails, the calculations have to be performed with each access.
In RAID 5, where there is only one parity block per stripe, the failure of a second drive results in total data loss.
The maximum number of drives is theoretically unlimited, but it is common practice to keep the maximum to 14 or fewer for RAID 5 implementations which have only one parity block per stripe. The reason for this restriction is that there is a greater likelihood of two drives in an array failing in rapid succession when there is greater number of drives. As the number of disks in a RAID 5 increases, the MTBF for the array as a whole can even become lower than that of a single disk. This happens when the likelihood of a second disk failing out of (N-1) dependent disks, within the time it takes to detect, replace and recreate a first failed disk, becomes larger than the likelihood of a single disk failing.
One should be aware that many disks together increase heat, which lowers the real-world MTBF of each disk. Additionally, a group of disks bought at the same time may reach the end of their Failure_rate#Bathtub_curve together, noticeably lowering the effective MTBF of the disks during that time.
In implementations with greater than 14 drives, or in situations where extreme redundancy is needed, RAID 5 with dual parity (also known as RAID 6) is sometimes used, since it can survive the failure of two disks.
== RAID 6 ==
A RAID 6 extends RAID 5 by adding an additional parity block, thus it uses block-level striping with two parity blocks distributed across all member disks. It was not one of the original RAID levels.
RAID 5 can be seen as a special case of a Reed-Solomon code where the syndrome used is the one built from generator 1 . Thus RAID 5 only requires addition in the galois field. Since we are operating on bytes, the field used is a binary galois field (GF(2^m)), typically of order 8. In binary galois fields, addition is computed by a simple XOR.
After understanding RAID 5 as a special case of a Reed-Solomon code, it is easy to see that it is possible to extend the approach to produce redundancy simply by producing another syndrome using a different generator; for example, 2 in GF(2^8). By adding additional generators it is possible to achieve any number of redundant disks, and recover from the failure of that many drives anywhere in the array.
Like RAID 5 the parity is distributed in stripes, with the parity blocks in a different place in each stripe.
Traditional Typical RAID 5 RAID 6 A1 A2 A3 Ap A1 A2 A3 Ap Aq B1 B2 Bp B3 B1 B2 Bp Bq B3 C1 Cp C2 C3 C1 Cp Cq C2 C3 Dp D1 D2 D3 Dp Dq D1 D2 D3
Note: A1, B1, et cetera each represent one data block. p and q represent the two Reed-Solomon syndromes.
RAID 6 is inefficient when used with a small number of drives but as arrays become bigger and have more drives the loss in storage capacity becomes less important and the probability of two disks failing at once becomes greater. RAID 6 provides protection against double disk failures and failures while a single disk is rebuilding. In the case where there is only one array it makes more sense than having a hot spare disk.
The user capacity of a RAID 6 array is n-2, where n is the total number of drives in the array.
RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations due to the overhead associated with the additional parity calculations.
= Nested RAID Levels =
Many storage controllers allow RAID levels to be nested. That is, one RAID can use another as its basic element, instead of using physical disks. You can think of these arrays as layered on top of each other, with physical disks at the bottom.
Nested RAIDs are usually signified by joining the numbers indicating the RAID levels into a single number, sometimes with a + in between. For example, RAID 10 (or RAID 1+0) conceptually consists of multiple level 1 arrays stored on physical disks with a level 0 array on top, striped over the level 1 arrays. In the case of RAID 0+1, it is most often called RAID 0+1 as opposed to RAID 01 to avoid confusion with RAID 1. Opposed to this, when the top array is a RAID 0 (such as in RAID 10 and RAID 50), most vendors choose to omit the + , probably because RAID 50 sounds fancier than the more explanatory RAID 5+0.
When nesting RAID levels, a RAID type that provides redundancy is typically combined with RAID 0 to boost performance. With these configurations it is preferable to have RAID 0 on top and the redundant array at the bottom, because fewer disks then need to be regenerated when a disk fails. (Thus, RAID 10 is for example preferable to RAID 0+1 but the administrative advantages of Splitting the Mirror of RAID1 would be lost).
== RAID 0+1 ==
A RAID 0+1 (also called RAID 01, although it shouldn t be confused with RAID 1) is a RAID used for both replicating and sharing data among disks. The difference between RAID 0+1 and RAID 10 is the location of each RAID system — it is a mirror of stripes. Consider an example of RAID 0+1: six 120 GB drives need to be set up on a RAID 0+1. Below is an example where two 360 GB level 0 arrays are mirrored, creating 360 GB of total storage space:
RAID 1 /-------------------------- | | RAID 0 RAID 0 /----------------- /----------------- | | | | | | 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB A1 A2 A3 A1 A2 A3 A4 A5 A6 A4 A5 A6 B1 B2 B3 B1 B2 B3 B4 B5 B6 B4 B5 B6
Note: A1, B1, et cetera each represent one data block.
where the maximum storage space here is 360 GB, spread across two arrays. The advantage is that when a hard drive fails in one of the level 0 arrays, the missing data can be transferred from the other array. However, adding an extra hard drive requires you to add an additional hard drive to balance out storage among the arrays.
It is not as robust as RAID 10 and cannot tolerate two simultaneous disk failures, if not from the same stripe. That is, once a single disk fails, each of the mechanisms in the other stripe is single point of failure. Also, once the single failed mechanism is replaced, in order to rebuild its data all the disks in the array must participate in the rebuild.
To add to the confusion, some controllers that run in RAID 0+1 mode combine the striping and mirroring into a single operation. The layout of the blocks for RAID 0+1 and RAID 10 are identical except that the disks are in a different order. To the smart controller this does not matter and they gain all the benefits of RAID 10 but are still labelled as only supporting RAID 0+1 in their documentation.
== RAID 10 ==
A RAID 10, sometimes called RAID 1+0, or RAID 1&0, is similar to a RAID 0+1 with exception that the RAID levels used are reversed—RAID 10 is a stripe of mirrors. Below is an example where three collections of 120 GB level 1 arrays are striped together to add up to 360 GB of total storage space:
RAID 0 /----------------------------------- | | | RAID 1 RAID 1 RAID 1 /-------- /-------- /-------- | | | | | | 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6 B1 B1 B2 B2 B3 B3 B4 B4 B5 B5 B6 B6
Note: A1, B1, et cetera each represent one data block.
One drive from each RAID 1 set could fail without damaging the data. However, if the failed drive is not replaced, the single working hard drive in the set then becomes a single point of failure for the entire array. If that single hard drive then fails, all data stored in the entire array is lost.
Extra 120GB hard drives could be added to any one of the level 1 arrays to provide extra redundancy. Unlike RAID 0+1, all the sub-arrays do not have to be upgraded simultaneously.
RAID10 is often the primary choice for high-load databases, because the lack of parity to calculate gives it faster write speeds.
== RAID 50 (RAID 5+0)==
A RAID 50 combines the block-level striping with distributed parity of RAID 5, with the straight block-level striping of RAID 0. This is a RAID 0 array striped across RAID 5 elements.
Below is an example where three collections of 120 GB RAID 5 s are striped together to add up to 720 GB of total storage space:
RAID 0 /----------------------------------------------------- | | | RAID 5 RAID 5 RAID 5 /----------------- /----------------- /----------------- | | | | | | | | | 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB A1 A2 Ap A3 A4 Ap A5 A6 Ap B1 Bp B2 B3 Bp B4 B5 Bp B6 Cp C1 C2 Cp C3 C4 Cp C5 C6 D1 D2 Dp D3 D4 Dp D5 D6 Dp
Note: A1, B1, et cetera each represent one data block; Ap, Bp, et cetera each represent parity information for each distinct RAID 5 and may represent different values across the RAID 0 (that is, Ap for A1 and A2 can differ from Ap for A3 and A4).
One drive from each of the RAID sets could fail without damaging the data. However, if the failed drive is not replaced, the remaining working drives in that set then become a single point of failure for the entire array. If one of those drives then fail, all data stored in the entire array is lost. The time spent in recovery (detecting and responding to a drive failure, and the rebuild process to the newly inserted drive) represents a period of vulnerability to the RAID set.
In the example below, datasets may be striped across both RAID sets. A dataset with 5 blocks would have 3 blocks written to the first RAID set, and the next 2 blocks written to RAID set 2. RAID Set 1 RAID Set 2 A1 A2 A3 Ap A4 A5 A6 Ap B1 B2 Bp B3 B4 B5 Bp B6 C1 Cp C2 C3 C4 Cp C5 C6 Dp D1 D2 D3 Dp D4 D5 D6
Note: A1, B1, et cetera each represent one data block.
The configuration of the RAID sets will impact the overall fault tolerancy. A construction of three seven-drive RAID 5 sets has higher capacity and storage efficiency, but can only tolerate three maximum potential drive failures. A construction of seven three-drive RAID 5 sets can handle as many as seven drive failures but has lower capacity and storage efficiency.
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 positioning 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.
= 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[http://www.netapp.com/library/tr/3298.pdf]. 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.
This is more easily illustrated using RAID 4, Twin Syndrome RAID 4 (RAID 6 with a RAID 4 layout which is not actually implemented), 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.
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.
Running in degraded mode with a double parity system is not advised.
== 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 cache 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.
Traditional EMC RAID 5 RAID S A1 A2 A3 Ap A1 B1 C1 1p B1 B2 Bp B3 A2 B2 C2 2p C1 Cp C2 C3 A3 B3 C3 3p Dp D1 D2 D3 A4 B4 C4 4p
Note: A1, B1, et cetera each represent one data block. A, B, et cetera are entire volumes.
== 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 A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6 This is obviously equivalent to the standard RAID 10 arrangement when k does divide n .
== Reliability of RAID configurations ==
Failure rate: The mean time to failure of a given RAID may be lower or higher than those of its constituent hard drives, depending on what type of RAID is employed.
Mean time to data loss (MTTDL): In this context, the mean time to elapse before a loss of user data in a given array, usually measured in hours.
Mean time to recovery (MTTR): In arrays that include redundancy for reliability, this is the time following a failure to restore an array to its normal failure-tolerant mode of operation. This includes time to replace a failed disk mechanism as well as time to re-build the array (i.e. to replicate data for redundancy).
= See also =
= References =
#H. Peter Anvin, The mathematics of RAID-6 . [http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf (online paper)]
= External links =
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