Standard RAID levels

In certain RAID levels (all but 0, 1, and 2), redundancy is achieved by the use of parity blocks. If a single drive in the array fails, data blocks and a parity block from the working drives can be combined to reconstruct the missing data.
Given the diagram below, assume A1 = 00000111, A2 = 00000101, and A3 = 00000000. Ap, generated by XORing A1, A2, and A3, will then equal 00000010. If the second drive fails, A2 will no longer be accessible, but can be reconstructed by XORing A1, A3, and Ap

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 (ie. 2 times the size of the smallest: 100GB). 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 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. 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, limited only by the speed of the RAID controller.
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 & newer to mount partitions under directories, much like Unix, and hence 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, data integrity is not very important, but cost is a consideration to most users. However, since data is shared between drives without redundancy, hard drives cannot be swapped out as all disks are dependent upon each other.

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 concatenated together, end to beginning, so they appear to be a single large disk.
In this sense, concatenation is akin to 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 (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.
JBOD is similar to the widely used Logical Volume Manager (LVM) and Logical Storage Manager (LSM) in UNIX and UNIX-based operating systems (OS). JBOD is useful for OSs which do not support LVM/LSM (like MS-Windows, although Windows 2003 Server, Windows XP Pro, and Windows 2000 support software JBOD, known as spanning dynamic disks). The difference between JBOD and LVM/LSM is that the address remapping between the logical address of the concatenated device and the physical address of the disc is done by the RAID hardware instead of the OS kernel as it is LVM/LSM.
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. However, JBOD does not carry the performance benefits which are associated with RAID 0.

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 is more important than data capacity. Such an array can only be as big as the smallest member disk. A classic RAID 1 mirrored pair contains two disks, 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. For example, consider a model of disk drive with a weekly probability of failure of 1:500. Assuming defective drives are replaced weekly, a two-drive RAID 1 installation would carry a 1:250,000 probability of failure for a given week. That is, the chances that both drives experience an ordinary mechanical failure during the same week is the square of the chances for only one drive.
Additionally, since all the data exists in two or more copies, each with its own hardware, the read performance goes up roughly as a linear multiple of the number of copies. That is, a RAID 1 array of three drives can be reading in three different places at the same time. 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. 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 unlike RAID 0. 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. Many older IDE RAID 1 cards read 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 commercial availability. When writing, the array performs like a single disk as all mirrors must be written with the data.
RAID 1 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 requires that the application support 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 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.

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. Extremely high data transfer rates are possible.

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-A9, 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.

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.
In our example above, 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.

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.
In the example above, a read request for block "A1" would be serviced by disk 1. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently.
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, 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 a single parity block per stripe, the failure of a second drive results in total data loss.
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 MTBF 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 run in a redundancy group to minimize the odds of simultaneous early life and end of life failures as evidenced by the bathtub curve.
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.
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 other techniques are commonly used to reduce the window of vulnerability of this occurring.

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 requires only addition in the galois field. Since we are operating on bytes, the field used is a binary galois field (GF\left(2^m\right)), 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\left(2^8\right). 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.
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 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.

Jagath Krishnakumar