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RAID

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For other uses, see Raid.

In computing, the acronym RAID (originally redundant array of inexpensive disks, now also known as redundant array of independent disks) refers to a data storage scheme using multiple hard drives to share or replicate data among the drives. Depending on the configuration of the RAID (typically referred to as the RAID level), 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, its key advantage was the ability to combine multiple low-cost devices using older technology into an array that offered greater capacity, reliability, 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 a 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 (but not necessarily) 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 personal 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 object to RAID 0 being called a RAID at all. 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 combine physical disk space for purposes of reliability, capacity, or performance (or even sociability — see the Just a Bunch Of Disks discussion) is a RAID system.

History

Norman Ken Ouchi at IBM was awarded U.S. patent 4,092,732 titled "System for recovering data stored in failed memory unit" in 1978 and the claims for this patent describe what would later be termed RAID 5 with full stripe writes. This 1978 patent also mentions that disk mirroring or duplexing (what would later be termed RAID 1) and protection with dedicated parity (that would later be termed RAID 4) were prior art at that time.

RAID technology was first defined by a group of computer scientists at the University of California, Berkeley in 1987. The scientists studied the possibility of using two or more disks to appear as a single device to the host system. [1]

In 1987, RAID levels 1 through 5 were formally defined by David A. Patterson, Garth A. Gibson and Randy H. Katz in the paper, "A Case for Redundant Arrays of Inexpensive Disks (RAID)". This was publicly published in the SIGMOD Conference 1988: pp 109–116. The term "RAID" was first introduced in this paper; it spawned the entire disk array industry.

RAID implementations

Hardware vs. software

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The distribution of data across multiple disks can be managed by either dedicated hardware or by software. Additionally, there are hybrid RAIDs that are partially software AND hardware-based solutions.

With a software implementation, the operating system manages the disks of the array through the normal drive controller (IDE/ATA, SATA, SCSI, Fibre Channel, etc.). With present CPU speeds, software RAID can be faster than hardware RAID[citation needed], 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 back cache which can speed up an application, such as an OLTP database server. In this case, the hardware RAID implementation flushes the write cache to secure storage to preserve data at a known point if there is a crash. The hardware approach is faster than accessing the disk drive and limited by RAM speeds, the rate at which the cache can be mirrored to another controller, 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, ATA, SATA, SCSI, Fibre Channel, or any combination thereof. The controller links to the host computer(s) with one or more high-speed SCSI, PCIe, Fibre Channel or iSCSI 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. Hybrid RAIDs have become very popular with the introduction of inexpensive hardware RAID controllers. The hardware is a normal disk controller that has no RAID features, but there is a boot-time application that allows users to set up RAIDs that are controlled via the BIOS. When any modern operating system is used, it will need specialized RAID drivers that will make the array look like a single block device. Since these controllers actually do all 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 drive that has failed. This reduces the mean time to repair period during which a second drive failure in the same RAID redundancy group can result in loss of data.

Some software RAID systems allow one to build 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 need to be RAID.

Standard RAID levels

A quick summary of the most commonly used RAID levels:

Common nested RAID levels:

  • RAID 01: A mirror of stripes
  • RAID 10: A stripe of mirrors
  • RAID 30: A stripe across dedicated parity RAID systems
  • RAID 100: A stripe of a stripe of mirrors

RAID parity blocks

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:

A1 XOR A3 XOR Ap = 00000101 (A2). 

       RAID
 A1        A2        A3      Ap
 B1        B2        Bp      B3
 C1        Cp        C2      C3 
 Dp        D1        D2      D3

Note: Data blocks are in the format A#, parity blocks Ap.

RAID 0

Diagram of a RAID 0 setup.

A RAID 0 (also known as a stripe set or striped volume) 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 (i.e. 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, and recovery of drive data is very costly.

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 filesystem 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 non raid 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 then the apparent seek time of the array will be half that of a single non raid drive (assuming identical disks in the array). 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, 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.

NOTE: Some sites have stated that for home PCs, the speed advantages are debatable. [2] [3]

Concatenation (JBOD)

Diagram of a JBOD setup.

Although a concatenation of disks (also called JBOD, or "Just a Bunch Of Drives") 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 Server 2003, 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.

Note: Some Raid cards (Ex. 3ware) use JBOD to refer to configuring drives without raid features including concatenation. Each drive shows up separately in the OS.

Note: Many Linux distributions refer to JBOD as "linear mode" or "append mode." The Mac OS 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 do have the write performance documented in the illustration above.

RAID 1

Diagram of a RAID 1 setup.

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 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.

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 discs this would double the transfer rate. The apparent access time of the array would be half that of a single non raid 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 add drives to the mirror. Three disks would give you three times the throughput and a apparent seek time of a third. 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. Note that these performance scenarios are in the best case with optimal access patterns.

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.

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.

Theoretically, RAID 2 would require 39 disks in a modern computer environment; 32 disks would be used for storage of the individual bits making up each word, and 7 would be used for error correction. The use of Hamming codes also permits using 7 disks in RAID 2, with 4 being used for data storage and 3 being used for error correction.

RAID 3

Diagram of a RAID 3 setup. Each number represents one data byte; each column represents one disk.

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

Diagram of a RAID 4 setup. Each number represents one data block; each column represents one disk.

A RAID 4 uses block-level striping with a dedicated parity disk. 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, 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

Diagram of a RAID 5 setup.

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 disk are required in a RAID 5 configuration.

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. Whereas The MBR table is written separately in all the Physical drives.

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 (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.

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 discs. If you ignore the parity blocks the on disc layout looks exactly like that of 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 other techniques are commonly used to reduce the window of vulnerability of this occurring.

RAID 6

Diagram of a RAID 6 setup. Each number represents one data block; each column represents one disk; p and q represent the two Reed-Solomon syndromes.

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 5, 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 ( 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.

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 user capacity of a RAID 6 array is (n-2)* x, where n is the total number of drives in the array and x 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.

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."

RAID 5E, RAID 5EE and RAID 6E

RAID 5E, RAID 5EE and RAID 6E generally refer to variants of RAID 5 or RAID 6 with online (hot) spare drives, where the spare drives are an active part of the block rotation scheme. This allows the I/O to be spread across all drives, including the spare, thus reducing the I/O bandwidth per drive, allowing for higher performance. It does, however, mean that a spare drive cannot be shared among multiple arrays, which is occasionally desirable.

In RAID 5E, RAID 5EE and RAID 6E, there is no dedicated "spare drive", just like there is no dedicated "parity drive" in RAID 5 or RAID 6. Instead, the spare blocks are distributed across all the drives, so that in a 10-disk RAID 5E with one spare, each and every disk is 80% data, 10% parity, and 10% spare.The spare blocks in RAID 5E and RAID 6E are at end of array, while in RAID 5EE the spare blocks are integrated into the array.RAID 5EE level can sustain a single drive failure.RAID 5EE requires at least four disks and can expand up to 16 disks.

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. It is instructive to 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. However, when the top array is a RAID 0 (such as in RAID 10 and RAID 50), most vendors choose to omit the '+', though RAID 5+0 is more informative.

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 preferable to RAID 0+1 but the administrative advantages of "splitting the mirror" of RAID 1 would be lost).

RAID 0+1

Block diagram of a RAID 0+1 setup.

A RAID 0+1 (also called RAID 01, not to 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 1+0 is the location of each RAID system — RAID 0+1 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
  A7       A8       A9       A7       A8       A9
  A10      A11      A12      A10      A11      A12

Note: A1, A2, et cetera each represent one data block; each column represents one disk.

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 to one stripe requires you to add an additional hard drive to the other stripes to balance out storage among the arrays.

It is not as robust as RAID 10 and cannot tolerate two simultaneous disk failures, unless the second failed disk is from the same stripe as the first. 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.

The exception to this is if all the disks are hooked up to the same raid controller in which case the controller can do the same error recovery as RAID 10 as it can still access the functional disks in each RAID 0 set. If you compare the diagrams between RAID 0+1 and RAID 10 and ignore the lines above the disks you will see that all that's different is that the disks are swapped around. If the controller has a direct link to each disk it can do the same. In this one case there is no difference between RAID 0+1 and RAID 10.

With increasingly larger capacity disk drives (driven by serial ATA drives), the risk of drive failure is increasing. Additionally, bit error correction technologies have not kept up with rapidly rising drive capacities, resulting in higher risks of encountering media errors. In the case where a failed drive is not replaced in a RAID0+1 configuration, a single uncorrectable media error occurring on the mirrored hard drive would result in data loss.

Given these increasing risks with RAID0+1, many business and mission critical enterprise environments are beginning to evaluate more fault tolerant RAID setups that add underlying disk parity. Among the most promising are hybrid approaches such as RAID0+1+5 (mirroring above single parity) or RAID0+1+6 (mirroring above dual parity).

RAID 10

Diagram of a RAID 10 setup.

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 make 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
  A7       A7       A8       A8       A9       A9
  A10      A10      A11      A11      A12      A12

Note: A1, A2, et cetera each represent one data block; each column represents one disk.

All but 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. As is the case with RAID 0+1, if a failed drive is not replaced in a RAID 10 configuration then a single uncorrectable media error occurring on the mirrored hard drive would result in data loss.

Given these increasing risks with RAID 10, many business and mission critical enterprise environments are beginning to evaluate more fault tolerant RAID setups that add underlying disk parity. Among the most promising are hybrid approaches such as RAID 0+1+5 (mirroring above single parity) or RAID 0+1+6 (mirroring above dual parity).

RAID 10 is often the primary choice for high-load databases, because the lack of parity to calculate gives it faster write speeds.

RAID 10 Capacity: (Size of Smallest Drive) * (Number of Drives) / 2


RAID 30

RAID level 30 is also known as striping of dedicated parity arrays. It is a combination of RAID level 3 and RAID level 0. RAID 30 provides high data transfer rates, combined with high data reliability. RAID 30 is best implemented on two RAID 3 disk arrays with data striped across both disk arrays. RAID 30 breaks up data into smaller blocks, and then stripes the blocks of data to each RAID 3 raid set. RAID 3 breaks up data into smaller blocks, calculates parity by performing an Exclusive OR on the blocks, and then writes the blocks to all but one drive in the array. The parity bit created using the Exclusive OR is then written to the last drive in each RAID 3 array. The size of each block is determined by the stripe size parameter, which is set when the RAID is created.

Advantages

One drive from each of the underlying RAID 3 sets can fail. Until the failed drives are replaced the other drives in the sets that suffered such a failure are a single point of failure for the entire RAID 30 array. In other words, if one of those drives fails, 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.

Offers highest level of redundancy and performance

Disadvantages

Very costly to implement 

           /------/------/------/------> RAID CONTROLLER <------\-------\------\------\
           |      |      |      |                           |       |      |       |
         disk1  disk2   disk3  disk4                      disk5   disk6   disk7  disk 8
           |      |      |      |                           |       |      |       |
          A1      A2     A3     P                          A4       A5    A6      P1
           |      |      |      |                           |       |      |       | 
          A7      A8     A9     P                          A10      A11   A12     P1
           |      |      |      |                           |       |      |       |
          A13     A14   A15     P                          A16     A17    A18     P1
           -------RAID 3---------                          ----------RAID 3---------
                 ----------------------- RAID 0 ----------------------------

RAID 100 (RAID 10+0)

A RAID 100, sometimes also called RAID 10+0, is a stripe of RAID 10s. RAID 100 is an example of plaid RAID, a RAID in which striped RAIDs are themselves striped together. Below is an example in which two sets of four 120 GB RAID 1 arrays are striped and re-striped to make 480 GB of total storage space: 

                                RAID 0
               .-------------------------------------.
               |                                     |
             RAID 0                                RAID 0
       .-----------------.                  .-----------------.
       |                 |                  |                 |
     RAID 1            RAID 1             RAID 1            RAID 1
   .--------.        .--------.         .--------.        .--------.
   |        |        |        |         |        |        |        |
120 GB   120 GB   120 GB   120 GB    120 GB   120 GB   120 GB   120 GB
  A1       A1       A2       A2        A3       A3       A4       A4
  A5       A5       A6       A6        A7       A7       A8       A8
  B1       B1       B2       B2        B3       B3       B4       B4
  B5       B5       B6       B6        B7       B7       B8       B8

Note: A1, B1, et cetera each represent one data sector; each column represents one disk.

All but one drive from each RAID 1 set could fail without loss of data. However, the remaining disk from the RAID 1 becomes a single point of failure for the already degraded array. Often the top level stripe is done in software. Some vendors call the top level stripe a MetaLun, or a Soft Stripe.

The major benefits of RAID 100 (and plaid RAID in general) over single-level RAID are better random read performance and the mitigation of hotspot risk on the array. For these reasons, RAID 100 is often the best choice for very large databases, where the underlying array software limits the amount of physical disks allowed in each standard array. Implementing nested RAID levels allows virtually limitless spindle counts in a single logical volume.

RAID 50 (RAID 5+0)

A RAID 50 combines the straight block-level striping of RAID 0 with the distributed parity of RAID 5. This is a RAID 0 array striped across RAID 5 elements.

Below is an example where three collections of 240 GB RAID 5s are striped together to make 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; each column represents one disk; 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 5 sets could fail without loss of data. However, if the failed drive is not replaced, the remaining drives in that set then become a single point of failure for the entire array. If one of those drives fails, 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

The configuration of the RAID sets will impact the overall fault tolerance. A construction of three seven-drive RAID 5 sets has higher capacity and storage efficiency, but can only tolerate three maximum potential drive failures. Because the reliability of the system depends on quick replacement of the bad drive so the array can rebuild, it is common to construct three six-drive RAID5 sets each with a hot spare that can immediately start rebuilding the array on failure. This does not address the issue that the array is put under maximum strain reading every bit to rebuild the array precisely at the time when it is most vulnerable. 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[citation needed], 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.

RAID 60 (RAID 6+0)

A RAID 60 combines the straight block-level striping of RAID 0 with the distributed double parity of RAID 6. That is, a RAID 0 array striped across RAID 6 elements.It requires at least 8 disks.

Below is an example where two collections of 240 GB RAID 6s are striped together to make 480 GB of total storage space: 

                                RAID 0
                .------------------------------------.
                |                                    |
              RAID 6                               RAID 6
   .--------------------------.        .--------------------------.
   |        |        |        |        |        |        |        |
120 GB   120 GB   120 GB   120 GB   120 GB   120 GB   120 GB   120 GB
  A1       A2       Aq       Ap       A3       A4       Aq       Ap
  B1       Bq       Bp       B2       B3       Bq       Bp       B4
  Cq       Cp       C1       C2       Cq       Cp       C3       C4
  Dp       D1       D2       Dq       Dp       D3       D4       Dq


As it is based on RAID 6, two disks from each of the RAID 6 sets could fail without loss of data.Also failures while a single disk is rebuilding in one RAID 6 set will not lead to data loss.RAID 60 has improved fault tolerance and it is quite impossible to lose data as more than half of the disks must fail in the above example in order to lose data.

Striping helps to increase capacity and performance without adding disks to each RAID 6 set (which would decrease data availability and could impact performance).RAID 60 improves upon the performance of RAID 6.Despite that RAID 60 is slightly worse than RAID 50 in terms of writes due to the added overhead of more parity calculations, but may be slightly faster in random reads due to the spreading of data over at least one more disk per RAID 6 set.When data security is concerned this performance drop is negligible.

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

Diagram of a RAID DP (Double Parity) setup.

One common addition to the existing RAID levels is double parity, sometimes implemented and known as diagonal parity[4]. 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; each column represents one disk.

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 caching 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; each column represents one disk.
      A, B, et cetera are entire volumes.

Matrix RAID

Diagram of a Matrix RAID setup.

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.

Linux can also create other RAID configurations using the md driver (0, 1, 4, 5, 6) as well as non-raid uses like multipath and LVM2. This md driver should not be confused with the dm driver, which is for IDE/ATA chipset based software raid (i.e., fakeraid).

IBM ServeRAID 1E

Diagram of a RAID 1E setup.

The IBM ServeRAID adapter series supports 2-way mirroring on an arbitrary number of drives.

This configuration is tolerant of non-adjacent drives failing. Other storage systems including Sun's StorEdge T3 support this mode as well.


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RAID-Z

Sun's ZFS implements an integrated redundancy scheme similar to RAID 5 which it calls RAID-Z. RAID-Z avoids the RAID 5 "write hole" [5] by its copy-on-write policy: rather than overwriting old data with new data, it writes new data 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. There is also RAID-Z2 which uses two forms of parity to achieve similar results as RAID 6: the ability to lose up to two drives without losing data [6].

What RAID Can and Cannot Do

This guide was taken from a thread in a RAID-related forum to help clarify the advantages and disadvantages to choosing RAID for either increases in performance or redundancy. It contains links to other threads in its forum containing user-generated anecdotal reviews of their RAID experiences.

What RAID Can Do

  • RAID can protect uptime. RAID levels 1, 0+1/10, 5, and 6 (and their variants such as 50 and 51) allow a mechanical hard disk to fail while keeping the data on the array accessible to users. Rather than being required to perform a time consuming restore from tape, DVD, or other slow backup media, RAID allows data to be restored to a replacement disk from the other members of the array, while being simultaneously available to users in a degraded state. This is of high value to enterprises, as downtime quickly leads to lost earning power. For home users, it can protect uptime of large media storage arrays, which would require time consuming restoration from dozens of DVD or quite a few tapes in the event of a disk failing that is not protected by redundancy.
  • RAID can increase performance in certain applications. RAID levels 0, and 5-6 all use variations on striping, which allows multiple spindles to increase sustained transfer rates when conducting linear transfers. Workstation type applications that work with large files, such as image and video editing applications, benefit greatly from disk striping. The extra throughput offered by disk striping is also useful in disk-to-disk backups applications. Also if RAID 1 or a striping based RAID with a sufficiently large block size is used RAID can provide performance improvements for access patterns involving multiple simultaneous random accesses (e.g., multi-user databases).

What RAID Cannot Do

  • RAID cannot protect the data on the array. A RAID array has one file system. This creates a single point of failure. A RAID array's file system is vulnerable to a wide variety of hazards other than physical disk failure, so RAID cannot defend against these sources of data loss. RAID will not stop a virus from destroying data. RAID will not prevent corruption. RAID will not save data from accidental modification or deletion by the user. RAID does not protect data from hardware failure of any component besides physical disks. RAID does not protect data from natural or man made disaster such as fires and floods. To protect data, data must be backed up to removable media, such as DVD, tape, or an external hard drive, and stored in an off site location. RAID alone will not prevent a disaster from turning into data loss. Disaster is not preventable, but backups allow data loss to be prevented.
  • RAID cannot simplify disaster recovery. When running a single disk, the disk is usually accessible with a generic ATA or SCSI driver built into most operating systems. However, most RAID controllers require specific drivers. Recovery tools that work with single disks on generic controllers will require special drivers to access data on RAID arrays. If these recovery tools are poorly coded and do not allow providing for additional drivers, then a RAID array will probably be inaccessible to that recovery tool.
  • RAID cannot provide a performance boost in all applications. This statement is especially true with typical desktop application users and gamers. Most desktop applications and games place performance emphasis on the buffer strategy and seek performance of the disk(s). Increasing raw sustained transfer rate shows little gains for desktop users and gamers, as most files that they access are typically very small anyway. Disk striping using RAID 0 increases linear transfer performance, not buffer and seek performance. As a result, disk striping using RAID 0 shows little to no performance gain in most desktop applications and games, although there are exceptions. For desktop users and gamers with high performance as a goal, it is better to buy a faster, bigger, and more expensive single disk than it is to run two slower/smaller drives in RAID 0. Even running the latest, greatest, and biggest drives in RAID-0 is unlikely to boost performance more than 10%, and performance may drop in some access patterns, particularly games.
  • RAID is not readily moved to a new system. When using a single disk, it is relatively straightforward to move the disk to a new system. Simply connect it to the new system, provided it has the same interface available. However, this is not so easy with a RAID array. A RAID BIOS must be able to read metadata from the array members in order to successfully construct the array and make it accessible to an operating system. Since RAID controller makers use different formats for their metadata (even controllers of different families from the same manufacturer may use incompatible metadata formats) it is virtually impossible to move a RAID array to a different controller. When moving a RAID array to a new system, plans should be made to move the controller as well. With the popularity of motherboard integrated RAID controllers, this is extremely difficult to accomplish. Generally, it is possible to move the RAID array members and controllers as a unit, and software RAID in Linux and Windows Server Products can also work around this limitation, but software RAID has other limitations (mostly performance related).

Reliability of RAID configurations

Failure rate
The mean time to failure (MTTF) or the mean time between failure (MTBF) 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 average time before a loss of data in a given array.
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).
Unrecoverable bit error rate (UBE)
This is the rate at which a disk drive will be unable to recover data after application of cyclic redundancy check (CRC) codes and multiple retries. This failure will present as a sector read failure. Some RAID implementations protect against this failure mode by remapping the bad sector, using the redundant data to retrieve a good copy of the data, and rewriting that good data to the newly mapped replacement sector. The UBE rate is typically specified at 1 bit in 1015 for enterprise class disk drives (SCSI, FC, SAS) , and 1 bit in 1014 for desktop class disk drives (IDE, ATA, SATA). Increasing disk capacities and large RAID 5 redundancy groups have led to an increasing inability to successfully rebuild a RAID group after a disk failure because an unrecoverable sector is found on the remaining disks. Double protection schemes such as RAID 6 are attempting to address this issue, but suffer from a very high write penalty.
Atomic Write Failure
Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, etc. This is a little understood and rarely mentioned failure mode for redundant storage systems. Database researcher Jim Gray wrote "Update in Place is a Poison Apple" during the early days of relational database commercialization. However, this warning largely went unheeded and fell by the wayside upon the advent of RAID, which many software engineers mistook as solving all data storage integrity and reliability problems. Many software programs update a storage object "in-place"; that is, they write a new version of the object on to the same disk addresses as the old version of the object. While the software may also log some delta information elsewhere, it expects the storage to present "atomic write semantics," meaning that the write of the data either occurred in its entirety or did not occur at all. However, very few storage systems provide support for atomic writes, and even fewer specify their rate of failure in providing this semantic. Note that during the act of writing an object, a RAID storage device will usually be writing all redundant copies of the object in parallel. Hence an error that occurs during the process of writing may leave the redundant copies in different states, and furthermore may leave the copies in neither the old nor the new state. The little known failure mode is that delta logging relies on the original data being either in the old or the new state so as to enable backing out the logical change, yet few storage systems provide an atomic write semantic on a raid disk.

See also

References

  1. ^ http://docs.info.apple.com/article.html?artnum=106594
  2. ^ H. Peter Anvin, "The mathematics of RAID-6". (online paper)

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