Standard RAID levels
The standard RAID levels are a basic set of RAID configurations that employ the techniques of striping, mirroring, or parity to create large reliable data stores from general purpose computer hard disk drives. The most common types today are RAID 0 (striping), RAID 1 and variants (mirroring), RAID 5 (distributed parity) and RAID 6 (dual parity). RAID levels and their associated data formats are standardized by the Storage Networking Industry Association in the Common RAID Disk Drive Format (DDF) standard.
A RAID 0 (also known as a stripe set or striped volume) splits data evenly across two or more disks (striped) without parity information for speed. RAID 0 was not one of the original RAID levels and provides no data redundancy. RAID 0 is normally used to increase performance, although it can also be used as a way to create a large logical disk out of two or more 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 100 GB disk is striped together with a 350 GB disk, the size of the array will be 200 GB (100 GB × 2).
The diagram shows how the data is distributed into Ax stripes to the disks. Accessing the stripes in the order A1, A2, A3, ... provides the illusion of a larger and faster drive. Once the stripe size is defined on creation it needs to be maintained at all times.
RAID 0 is also used in some computer gaming systems where performance is desired and data integrity is not very important. However, real-world tests with computer games have shown that RAID-0 performance gains are minimal, although some desktop applications will benefit. Another article examined these claims and concludes: "Striping does not always increase performance (in certain situations it will actually be slower than a non-RAID setup), but in most situations it will yield a significant improvement in performance." 
An exact copy (or mirror) of a set of data on two disks. This is useful when read performance or reliability is 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 reliability geometrically) over a single disk. Since each member contains a complete copy and can be addressed independently, ordinary wear-and-tear reliability is raised by the power of the number of self-contained copies.
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 most implementations of RAID 1 do not do this. 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 (for two disk arrays) or multiplexing (for arrays with more than two disks). 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, in theory, double the transfer rate when reading. 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 are present on all the disks. In reality, the need to move the drive heads to the next block (to skip blocks already read by the other drives) can effectively mitigate speed advantages for sequential access. Read performance can be further improved by adding drives to the mirror. Many older IDE RAID 1 controllers read only from one disk in the pair, so their read performance is always that of a single disk. Some older RAID 1 implementations read both disks simultaneously to compare the data and detect 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 are best case performance scenarios with optimal access patterns.
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 at the same angular orientation (they reach Index at the same time), so it generally cannot service multiple requests simultaneously. Extremely high data transfer rates are possible. This is the only original level of RAID that is not currently used.
All hard disks eventually implemented Hamming code error correction. This made RAID 2 error correction redundant and unnecessarily complex. This level quickly became useless and is now obsolete. There are no commercial applications of RAID 2.
A RAID 3 uses byte-level striping with a dedicated parity disk. RAID 3 is very rare in practice. One of the characteristics of RAID 3 is that it generally cannot service multiple requests simultaneously. This happens 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 and usually requires synchronized spindles.
This makes it suitable for applications that demand the highest transfer rates in long sequential reads and writes, for example uncompressed video editing. Applications that make small reads and writes from random disk locations will get the worst performance out of this level.
The requirement that all disks spin synchronously, a.k.a. lockstep, added design considerations to a level that didn't give significant advantages over other RAID levels, so it quickly became useless and is now obsolete. Both RAID 3 and RAID 4 were quickly replaced by RAID 5. RAID 3 was usually implemented in hardware, and the performance issues were addressed by using large disk caches.
In the example on the right, 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.
RAID 4 is very uncommon, but one enterprise level company that has previously used it is NetApp. The aforementioned performance problems were solved with their proprietary Write Anywhere File Layout (WAFL), an approach to writing data to disk locations that minimizes the conventional parity RAID write penalty. By storing system metadata (inodes, block maps, and inode maps) in the same way application data is stored, WAFL is able to write file system metadata blocks anywhere on the disk. This approach in turn allows multiple writes to be "gathered" and scheduled to the same RAID stripe—eliminating the traditional read-modify-write penalty prevalent in parity-based RAID schemes.
A RAID 5 comprises block-level striping with distributed parity. Unlike in RAID 4, parity information is distributed among the drives. It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.
RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations because of the overhead associated with parity calculations. Performance varies greatly depending on how RAID 6 is implemented in the manufacturer's storage architecture – in software, firmware or by using firmware and specialized ASICs for intensive parity calculations. It can be as fast as a RAID-5 system with one fewer drive (same number of data drives).
According to the Storage Networking Industry Association (SNIA), 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."
Two different syndromes need to be computed in order to allow the loss of any two drives. One of them, P can be the simple XOR of the data across the stripes, as with RAID 5. A second, independent syndrome is more complicated and requires the assistance of field theory.
To deal with this, the Galois field is introduced with , where for a suitable irreducible polynomial of degree . A chunk of data can be written as in base 2 where each is either 0 or 1. This is chosen to correspond with the element in the Galois field. Let correspond to the stripes of data across hard drives encoded as field elements in this manner (in practice they would probably be broken into byte-sized chunks). If is some generator of the field and denotes addition in the field while concatenation denotes multiplication, then and may be computed as follows ( denotes the number of data disks):
For a computer scientist, a good way to think about this is that is a bitwise XOR operator and is the action of a linear feedback shift register on a chunk of data. Thus, in the formula above, the calculation of P is just the XOR of each stripe. This is because addition in any characteristic two finite field reduces to the XOR operation. The computation of Q is the XOR of a shifted version of each stripe.
Mathematically, the generator is an element of the field such that is different for each nonnegative satisfying .
If one data drive is lost, the data can be recomputed from P just like with RAID 5. If two data drives are lost or a data drive and the drive containing P are lost, the data can be recovered from P and Q or from just Q, respectively, using a more complex process. Working out the details is extremely hard with field theory. Suppose that and are the lost values with . Using the other values of , constants and may be found so that and :
Multiplying both sides of the equation for by and adding to the former equation yields and thus a solution for , which may be used to compute .
The computation of Q is CPU intensive compared to the simplicity of P. Thus, a RAID 6 implemented in software will have a more significant effect on system performance, and a hardware solution will be more complex.
Non-standard RAID levels and non-RAID drive architectures
Alternatives to the above designs include nested RAID levels, non-standard RAID levels, and non-RAID drive architectures. Non-RAID drive architectures are referred to by similar acronyms, notably SLED, Just a Bunch of Disks, SPAN/BIG, and MAID.
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