Standard RAID levels
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The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring, or parity. The standard RAID levels can be nested for other benefits (see Nested RAID levels).
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[edit] Error-correction codes
For RAID 2 through 6 an error-correcting code is used to provide redundancy of the data.
For RAID 2, a Hamming code is used. For this level, extra disks are needed to store the error-correcting bits ("check disks" according to Patterson, et. al.).
RAID 3, 4 and 5 use standard parity bits by using the XOR logical function. For example, given the following three bytes:
- A1 = 00000111
- A2 = 00000101
- A3 = 00000000
Taking the XOR of all of these yields:
In terms of parity, the parity byte creates even parity. This means that the sum of 1s for each bit position yields an even number. In this example, the 2nd bit position from the right has one 1 while 1st and 3rd have two 1s; including the parity yields two (even) parity for these three bit positions.
The advantage of using parity is when one disk is lost (primary interest is when it is lost due to hardware failure). For example, say the disk containing A2 is lost leaving A1, A3, and Ap to reconstruct A2. This can be done by using the XOR operation again:
This value clearly matches the above definition of A2. This process can then be repeated for the remainder of the data.
All of the above examples use only three data bytes and one parity byte. (This is called a "stripe" in the remainder of this article and are given the same color in the diagrams.) When used in conjunction with RAID these operations happen on blocks (the fundamental operational/usable unit of storage in computer storage) instead of individual bytes.
[edit] Concatenation (JBOD or SPAN)
Concatenation or Spanning of disks is not one of the numbered RAID levels, but it is a popular method for combining multiple physical disk drives into a single virtual disk. As the name implies, disks are merely concatenated together, end to beginning, so they appear to be a single large disk. This mode is sometimes called JBOD, or "Just a Bunch Of Disks".
Concatenation may be thought of as the reverse of partitioning. 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 independent disks, it can be thought of as a distant relation to RAID. Concatenation is sometimes used to turn several odd-sized drives into one larger useful drive, which cannot be done with RAID 0. For example, JBOD could combine 3 GB, 15 GB, 5.5 GB, and 12 GB drives into a logical drive at 35.5 GB, which is often more useful than the individual drives separately.
In the diagram to the right, data is concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92). If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks, the size of the smallest disk in the array (disk 1) for a total size of 84 blocks.
Some RAID controllers use JBOD to refer to configuring drives without RAID features. Each drive shows up separately in the OS. This JBOD is not the same as concatenation.
Many Linux distributions use the terms incorrectly and refer to JBOD as "linear mode" or "append mode". The Mac OS X 10.4 implementation -- called a "Concatenated Disk Set" -- does not leave the user with any usable data on the remaining drives if one drive fails in a concatenated disk set, although the disks otherwise operate as described above.
[edit] RAID 0
A RAID 0 (also known as a stripe set or striped volume) splits data evenly across two or more disks (striped) with no parity information for redundancy. It is important to note that RAID 0 was not one of the original RAID levels and provides zero data redundancy. 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
In the diagram to the right, the odd blocks are written to disk 0 while the even blocks are written to disk 1 such that A1, A2, A3, A4, ... would be order of blocks read if read sequentially from the beginning.
[edit] RAID 0 failure rate
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, though the group reliability decreases with member size.
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 mean time to failure (MTTF) or mean time between failures (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 (in other words, the probability of a failure is roughly proportional to the number of members. If there were a probability of 5% that the disk would die within three years, in a two disk array, that probability would be upped to 1 − (1 − 0.05)2 = 0.0975 = 9.75%). 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 (the data cannot be recovered with the missing disk). Data can be recovered using special tools (see data recovery), however, this data will be incomplete and most likely corrupt, and recovery of drive data is very costly and not guaranteed. In the RAID-5 setup reliability is sought by mirroring the disk for emergency backups.
[edit] RAID 0 performance
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. How much the drives act independently depends on the access pattern from the file system level. For reads and writes that are larger than the stripe size, such as copying files or video playback, the disks will be seeking to the same position on each disk, so the seek time of the array will be the same as that of a single drive. For reads and writes that are smaller than the stripe size, such as database access, the drives will be able to seek to independently. If the sectors accessed are spread evenly between the two drives, the apparent seek time of the array will be half that of a single drive (assuming the disks in the array have identical access time characteristics). The transfer speed of the array will be the transfer speed of all the disks added together, limited only by the speed of the RAID controller. Note that these performance scenarios are in the best case with optimal access patterns.
RAID 0 is useful for setups such as large read-only NFS 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 disks. It is possible in Windows 2000 Professional and newer to mount partitions under directories, much like Unix, thus eliminating the need for a partition to be assigned a drive letter. RAID 0 is also a popular choice for gaming systems where performance is desired and data integrity is not very important. However, since data is shared between drives without redundancy, hard drives cannot be swapped out as all disks are dependent upon each other.
[edit] RAID 1
A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful when read performance or reliability are more important than data storage capacity. Such an array can only be as big as the smallest member disk. A classic RAID 1 mirrored pair contains two disks (see diagram), which increases reliability exponentially over a single disk. Since each member contains a complete copy of the data, and can be addressed independently, ordinary wear-and-tear reliability is raised by the power of the number of self-contained copies.
[edit] RAID 1 failure rate
For example, consider a RAID 1 with two identical models of a disk drive with a weekly probability of failure of 1:500. Assuming defective drives are replaced weekly, the installation would carry a 1:250,000 probability of failure for a given week. That is, the likelihood that the RAID array is down due to mechanical failure during any given week is the product of the likelihoods of failure of both drives. In other words, the probability of failure is 1 in 500 and if the failures are statistically independent then the probability of both drives failing is
.
This is purely theoretical however, the chance of a failure is much higher because drives are often manufactured at the same time and subjected to the same stresses. If a failure is because of an environmental problem, it's quite likely that the other drive will fail shortly after the first.
[edit] RAID 1 performance
Additionally, since all the data exists in two or more copies, each with its own hardware, the read performance can go up roughly as a linear multiple of the number of copies. That is, a RAID 1 array of two drives can be reading in two different places at the same time, though not all implementations of RAID 1 do this[1]. To maximize performance 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 and requested sectors can be split evenly between the disks. For the usual mirror of two disks, this would double the transfer rate. The apparent access time of the array would be half that of a single drive. Unlike RAID 0, this would be for all access patterns, as all the data is present on all the disks. Read performance can be further improved by adding drives to the mirror. Three disks would result in three times the throughput and an apparent seek time one third of that of a single drive. Many older IDE RAID 1 controllers read only from one disk in the pair, so their read performance is that of a single disk. Some older RAID 1 implementations would also read both disks simultaneously and compare the data to catch errors. The error detection and correction on modern disks makes this less useful in environments requiring normal availability. When writing, the array performs like a single disk, as all mirrors must be written with the data. Note that these performance scenarios are in the best case with optimal access patterns.
RAID 1 has many administrative advantages. For instance, in some 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 is useful in situations where the file system must be constantly available. This requires that the application supports recovery from the image of data on the disk at the point of the mirror split. This procedure is less critical in the presence of the "snapshot" feature of some file systems, in which some space is reserved for changes, presenting a static point-in-time view of the file system. Alternatively, a set of disks can be kept in much the same way as traditional backup tapes are.
[edit] 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 spin in perfect tandem. Extremely high data transfer rates are possible. This is the only original level of RAID that is not currently used.
The use of the Hamming(7,4) code (four data bits plus three parity bits) also permits using 7 disks in RAID 2, with 4 being used for data storage and 3 being used for error correction.
RAID 2 is the only standard RAID level which can repair and replace corrupt data (disks that return incorrect data on an attempted read), rather than just lost data (disks that return error on an attempted read.)
[edit] 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 above, a request for block "A" consisting of bytes A1-A6 would require all three data disks to seek to the beginning (A1) and reply with their contents. A simultaneous request for block B would have to wait.
[edit] RAID 4
A RAID 4 uses block-level striping with a dedicated parity disk. 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. RAID 4 looks similar to RAID 5 except that it does not use distributed parity, and similar to RAID 3 except that it stripes at the block level, rather than the byte level. Generally, RAID 4 is implemented with hardware support for parity calculations, and a minimum of 3 disks is required for a complete RAID 4 configuration.
In the example above, a read request for block "A1" would be serviced by disk 0. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently by disk 1.
[edit] RAID 5
A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 has achieved popularity due to its low cost of redundancy. Generally, RAID 5 is implemented with hardware support for parity calculations. A minimum of 3 disks is generally required for a complete RAID 5 configuration. A RAID 5 two disk set is possible, but many implementations do not allow for this. In some implementations a degraded disk set can be made (3 disk set of which 2 are online).
In the example above, a read request for block "A1" would be serviced by disk 0. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently by disk 1.
[edit] RAID 5 parity handling
Every time a block is written to a disk in a RAID 5, a parity block is generated within the same stripe. A block is often composed of many consecutive sectors on a disk. A series of blocks (a block 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. For small writes, this requires reading the old data, writing the new parity, and writing the new data. The disk used for the parity block is staggered from one stripe to the next, hence the term "distributed parity blocks". RAID 5 writes are expensive in terms of disk operations and traffic between the disks and the controller.
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: 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 a single parity block per stripe, the failure of a second drive results in total data loss.
[edit] RAID 5 disk failure rate
The maximum number of drives in a RAID 5 redundancy group is theoretically unlimited, but it is common practice to limit the number of drives. The tradeoffs of larger redundancy groups are greater probability of a simultaneous double disk failure, the increased time to rebuild a redundancy group, and the greater probability of encountering an unrecoverable sector during RAID reconstruction. As the number of disks in a RAID 5 group increases, the Mean Time Between Failures (MTBF, the reciprocal of the failure rate) can 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. RAID 6 is an alternative that provides dual parity protection thus enabling larger numbers of disks per RAID group.
Some RAID vendors will avoid placing disks from the same manufacturing lot in a redundancy group to minimize the odds of simultaneous early life and end of life failures as evidenced by the Bathtub curve.
[edit] RAID 5 performance
RAID 5 implementations suffer from poor performance when faced with a workload which includes many writes which are smaller than the capacity of a single stripe; this is because parity must be updated on each write, requiring read-modify-write sequences for both the data block and the parity block. More complex implementations often include non-volatile write back cache to reduce the performance impact of incremental parity updates.
The read performance of RAID 5 is almost as good as RAID 0 for the same number of disks. Except for the parity blocks, the distribution of data over the drives follows the same pattern as RAID 0. The reason RAID 5 is slightly slower is that the disks must skip over the parity blocks.
In the event of a system failure while there are active writes, the parity of a stripe may become inconsistent with the data. If this is not detected and repaired before a disk or block fails, data loss may ensue as incorrect parity will be used to reconstruct the missing block in that stripe. This potential vulnerability is sometimes known as the "write hole". Battery-backed cache and similar techniques are commonly used to reduce the window of opportunity for this to occur.
[edit] RAID 5 usable size
Parity data uses up the capacity of one drive in the array. (This can be seen by comparing it with RAID 4: RAID 5 distributes the parity data across the disks, while RAID 4 centralizes it on one disk, but the amount of parity data is the same.) In case that the drives vary in capacity, the smallest of them sets the bar. Therefore, the usable capacity of a RAID 5 array is (N − 1) * Smin, where N is the total number of drives in the array and Smin is the capacity of the smallest drive in the array.
The number of hard drives that can belong to a single array is theoretically unlimited (although the time required for initial construction of the array as well as that for reconstruction of a failed disk increases with the number of drives in an array).
[edit] 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[2]. RAID , being a degenerate case, requires only addition in the Galois field. Since we are operating on bits, the field used is a binary galois field 
. In cyclic representations of 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; typically a polynomial in 
(8 means we are operating on bytes). By adding additional syndromes it is possible to achieve any number of redundant disks, and recover from the failure of that many drives anywhere in the array, but RAID 6 refers to the specific case of two syndromes.
Like RAID 5, the parity is distributed in stripes, with the parity blocks in a different place in each stripe.
[edit] RAID 6 performance
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 is bigger. 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 may make more sense than having a hot spare disk.
The usable capacity of a RAID 6 array is 
, where N is the total number of drives in the array, Si is the capacity of the ith, and Smin is the capacity of the smallest drive 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. This penalty can be minimized by coalescing writes in fewer stripes, which can be achieved by a Write Anywhere File Layout.
[edit] RAID 6 implementation
According to SNIA (Storage Networking Industry Association), the definition of RAID 6 is: "Any form of RAID that can continue to execute read and write requests to all of a RAID array's virtual disks in the presence of any two concurrent disk failures. Several methods, including dual check data computations (parity and Reed Solomon), orthogonal dual parity check data and diagonal parity have been used to implement RAID Level 6."
[edit] Non-standard RAID levels
There are other RAID levels that are promoted by individual vendors, but not generally standardized. The non-standard RAID levels 5E, 5EE and 6E extend RAID 5 and 6 with hot-spare drives. Other non-standard RAID levels include RAID 1.5, RAID 7, RAID S or Parity RAID, Matrix RAID, RAID-Z, RAIDn, Linux MD RAID 10, and IBM ServeRAID 1E.
[edit] See also
[edit] References
- ^ http://docs.info.apple.com/article.html?artnum=106594
- ^ H. Peter Anvin, "The mathematics of RAID-6". (online paper)
