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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2007) |
For other uses, see Raid.
RAID - which stands for Redundant Arrays of Inexpensive Disks (as named by the inventors) or Redundant Arrays of Independent Disks (a name which later developed within the computing industry) - is a technology that employs the simultaneous use of two or more hard disk drives to achieve greater levels of performance, reliability, and/or larger data volume sizes.
The phrase "RAID" is an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. RAID\'s various designs all involve two key design goals: increased data reliability and increased input/output performance. When several physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across several disks, but the array is seen by the computer user or operating system as just one, single disk. RAID can be set up to serve several different purposes, the most common of which are outlined below.
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For redundancy purposes, extra "summary" data is written alongside the main data on a disk, so that the failure of one or more disks in an array will not result in data loss. The failed disk is replaced, and the data on it is reconstructed from the summary data on the other disks. Arrays used for this purpose allow less data to be stored, with the extra summary data requiring, on average, roughly half the size of the main data.
For increased performace, there are various combinations of configurations. The simplest of these, \'RAID 0\', splits data across two or more disks at once. This gives improved read/write speed and full disk capacity, but all of the written data is lost or corrupted if just one disk fails. Another approach, \'RAID 1\', stores the same data on each disk in the array so that the failure of one disk causes no loss. This configuration allows the user to consume only half the total capacity of the array\'s disks.
Another common configuration, \'RAID 5\' uses elements from both previously mentioned levels, to allow increased performance with some redundancy. Other levels do exist, and are discussed later in this article.
RAID involves significant computation when reading and writing information. Simpler and less expensive controllers may require the host computer\'s processor to do the computing, which reduces the computer\'s performance on processor-intensive tasks. Simpler RAID controllers may provide only levels 0 and 1, which require less processing.
RAID systems with redundancy can be designed to continue working without interruption when one, or sometimes more, disks of the array fail - bad disks can be hot swapped for new ones, and the array rebuilt while the system keeps running. Other systems have to be shut down while the data is recovered. RAID is often used in high availability systems, where it is important that the system keeps running as much of the time as possible.
RAID combines two or more physical hard disks into a single logical unit by using either special hardware or software. Hardware solutions often are designed to present themselves to the attached system as a single hard drive, and the operating system is unaware of the technical workings. Software solutions are typically implemented in the operating system, and again would present the RAID drive as a single drive to applications.
There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Different RAID levels use one or more of these techniques, depending on the system requirements. The main aims of using RAID are to improve reliability, important for protecting information that is critical to a business, for example a database of customer orders; or where speed is important, for example a system that delivers video on demand TV programs to many viewers.
The configuration affects reliability and performance in different ways. The problem with using more disks is that it is more likely that one will go wrong, but by using error checking the total system can be made more reliable by being able to survive and repair the failure. Basic mirroring can speed up reading data as a system can read different data from both the disks, but it may be slow for writing if the configuration requires that both disks must confirm that the data is correctly written. Striping is often used for performance, where it allows sequences of data to be read from multiple disks at the same time. Error checking typically will slow the system down as data needs to be read from several places and compared. The design of RAID systems is therefore a compromise and understanding the requirements of a system is important. Modern disk arrays typically provide the facility to select the appropriate RAID configuration.
A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary).
A brief summary of the most commonly used RAID levels. The SNIA Dictionary also contains definitions of the RAID levels that have been vetted by major storage industry players, and is referenced below as applicable.
| Level | Description | Minimum # of disks | Image |
|---|---|---|---|
| RAID 0 | Striped set without parity. Provides improved performance and additional storage but no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drive, the data is broken into fragments. The number of fragments is dictated by the number of disks in the drive. The fragments are written to their respective disks simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. When one sector on one of the disks fails, however, the corresponding sector on every other disk is rendered useless because part of the data is now corrupted. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the array means higher bandwidth, but greater risk of data loss. SNIA definition. | 2 | 
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| RAID 1 | Mirrored set without parity. Provides fault tolerance from disk errors and single disk failure. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning. SNIA definition. | 2 | 
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| RAID 3 | Striped set with dedicated parity. This mechanism provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single parity disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty. SNIA definition | 3 | 
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| RAID 4 | Identical to RAID 3 but does block-level striping instead of byte-level striping. SNIA definition | 3 | 
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| RAID 5 | Striped set with distributed parity. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. SNIA definition | 3 | 
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| RAID 6 | Striped set with dual parity. Provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high availability systems. This becomes increasingly important because large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if one drive, but no more, fails before the rebuild is complete. SNIA definition | 4 | 
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Many storage controllers allow RAID levels to be nested. That is a RAID array can be made of RAID arrays of any type, rather than disks. These arrays can be thought of as layered on top of each other, with physical drives at the bottom.
Nested RAIDs are usually described 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 drives 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 clearer.
Given the large amount of custom configurations available with a RAID array, many companies, organizations, and groups have created their own non-standard configurations, typically designed to meet the needs of at least one but usually very small niche groups of arrays. Most of these non-standard RAID levels are proprietary.
Some of the more prominent modifications are:
The distribution of data across multiple drives can be managed either by dedicated hardware or by software. When done in software the software may be part of the operating system or it may be part of the firmware and drivers supplied with the card.
Software implementations are now provided by many operating systems. A software layer sits above the (generally block-based) disk device drivers and provides an abstraction layer between the logical drives (RAID arrays) and physical drives. Most common levels are RAID 0 (striping across multiple drives for increased space and performance) and RAID 1 (mirroring two drives), followed by RAID 1+0, RAID 0+1, and RAID 5 (data striping with parity).
Microsoft\'s server operating systems support 3 RAID levels; RAID 0, RAID 1, and RAID 5. Microsoft desktop operating systems support RAID 0 only; there is no software support for fault-tolerant RAID levels in the desktop operating systems.
The software must run on a host server attached to storage, and server\'s processor must dedicate processing time to run the RAID software. This is negligible for RAID 0 and RAID 1, but may be significant for more complex parity-based schemes. Furthermore all the busses between the processor and the disk controller must carry the extra data required by RAID which may cause congestion.
Another concern with operating system-based RAID is the boot process, it can be difficult or impossible to set up the boot process such that it can failover to another drive if the usual boot drive fails and therefore such systems can require manual intervention to make the machine bootable again after a failure. Finally operating system-based RAID usually uses formats proprietary to the operating system in question so it cannot generally be used for partitions that are shared between operating systems as part of a multi-boot setup.
Most operating system-based implementations allow RAID arrays to be created from partitions rather than entire physical drives. For instance, an adminstrator could divide an odd number of disks into two partitions per disk, mirror partitions across disks and stripe a volume across the mirrored partitions to emulate a RAID 1E configuration. Using partitions in this way also allows mixing reliability levels on the same set of disks. For example, one could have a very robust RAID-1 partition for important files, and a less robust RAID-5 or RAID-0 partition for less important data. (Some high-end hardware controllers offer similar features, e.g. Intel Matrix RAID.) Using two partitions on the same drive in the same RAID array is, however, dangerous. If, for example, a RAID 5 array is composed of four drives 250 + 250 + 250 + 500 GB, with a 500 GB drive split into two 250 GB partitions, a failure of this drive will remove two partitions from the array, causing all of the data held on it to be lost.
Hardware RAID controllers use different, proprietary disk layouts, so it is not usually possible to span controllers from different manufacturers. They do not require processor resources, the BIOS can boot from them, and tighter integration with the device driver may offer better error handling.
A hardware implementation of RAID requires at least a special-purpose RAID controller. On a desktop system this may be a PCI expansion card, PCI-Express Expansion Card or built into the motherboard. Controllers supporting most types of drive may be used - IDE/ATA, SATA, SCSI, SSA, Fibre Channel, sometimes even a combination. The controller and disks may be in a stand-alone disk enclosure, rather than inside a computer. The enclosure may be directly attached to a computer, or connected via SAN. The controller hardware handles the management of the drives, and performs any parity calculations required by the chosen RAID level.
Most hardware implementations provide a read/write cache which, depending on the I/O workload, will improve performance. In most systems write cache may be non-volatile (e.g. battery-protected), so pending writes are not lost on a power failure.
Hardware implementations provide guaranteed performance, add no overhead to the local CPU complex and can support many operating systems, as the controller simply presents a logical disk to the operating system.
Hardware implementations also typically support hot swapping, allowing failed drives to be replaced while the system is running.
Operating system-based RAID cannot easily be used to protect the boot process and is generally impractical on desktop version of Windows (as described above). Hardware RAID controllers are expensive. To fill this gap companies introduced cheap "RAID controllers" that do not contain a RAID controller chip. Instead they consist of a standard disk controller chip with special firmware and drivers. During early stage bootup the RAID is implemented by the firmware, when a modern protected mode operating system such as Linux or Windows is loaded the drivers take over.
Since these controllers are sold as RAID controllers (a term which prior to their introduction implied a controller that did RAID in hardware) and it is rarely made clear to purchasers that the RAID is done in software they have become known as "fake RAID". Despite this name the RAID itself, while implemented in software, is real.
Main article: Network-Attached Storage
While not directly associated with RAID, Network-Attached Storage (NAS) is an enclosure containing disk drives and the equipment necessary to make them available over a computer network, usually Ethernet. The enclosure is basically a dedicated computer in its own right, designed to operate over the network without screen or keyboard. It contains one or more disk drives; multiple drives may be configured as a RAID array.
Both hardware and software implementations may support the use of hot spare drives, a pre-installed drive which is used to immediately (and automatically) replace a drive that has failed, by rebuilding the array onto that empty drive. 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, though it doesn\'t eliminate it completely; array rebuilds still take time, especially on active systems. This is especially important as the failure of drives in an array is unlikely to be completely independent. The drives will have received a similar load pattern so are likely to come into wear out failure at about the same time.
RAID 6 uses the same number of drives as RAID 5 with a hot spare and eliminates the window of vulnerability to a second drive failure mentioned above but requires a more advanced RAID controller.
The theory behind the error correction in RAID assumes that failures of drives are independent. Given these assumptions it is possible to calculate how often they can fail and to arrange the array to make data loss arbitrarily improbable.
In practice, the drives are often the same ages, with similar wear. Since many drive failures are due to mechanical issues which are more likely on older drives, this violates those assumptions and failures are in fact statistically correlated. In practice then, the chances of a second failure before the first has been recovered is not nearly as unlikely as might be supposed, and data loss can in practice occur at significant rates.Disk Failures in the Real World: What Does an MTTF of 1,000,000 Hours Mean to You? Bianca Schroeder and Garth A. Gibson
This is a little understood and rarely mentioned failure mode for redundant storage systems that do not utilize transactional features. 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, although overlapped or staggered writes are more common when a single RAID processor is responsible for multiple drives. 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.
While the battery-backed write cache may partially solve the problem, it is applicable only to a power failure scenario.
Since transactional support is not universally present in hardware RAID, many operating systems include transactional support to protect against data loss during an interrupted write. Novell Netware, starting with version 3.x, included a transaction tracking system. Microsoft introduced transaction tracking via the journalling feature in NTFS. NetApp WAFL file system solves it by never updating the data in place, as does ZFS.
This can 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 drives. Double protection schemes such as RAID 6 are attempting to address this issue, but suffer from a very high write penalty.
The disk system can acknowledge the write operation as soon as the data is in the cache, not waiting for the data to be physically written. However, any power outage can then mean a significant data loss of any data queued in such cache.
Often a battery is protecting the write cache, mostly solving the problem. If a write fails because of power failure, the controller may complete the pending writes as soon as restarted. This solution still has potential failure cases: the battery may have worn out, the power may be off for too long, the disks could be moved to another controller, the controller itself could fail. Some disk systems provide the capability of testing the battery periodically, which however leaves the system without a fully charged battery for several hours.
The disk formats on different RAID controllers are not necessarily compatible, so that it may not be possible to read a RAID array on different hardware. Consequently a non-disk hardware failure may require using identical hardware, or a data backup, to recover the data.
Norman Ken Ouchi at IBM was awarded a 1978 U.S. patent 4,092,732U.S. Patent 4,092,732 titled "System for recovering data stored in failed memory unit." 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.
The term RAID was first defined by David A. Patterson, Garth A. Gibson and Randy Katz at the University of California, Berkeley in 1987. They studied the possibility of using two or more drives to appear as a single device to the host system and published a paper: "A Case for Redundant Arrays of Inexpensive Disks (RAID)" in June 1988 at the SIGMOD conference. Patterson, David; Garth A. Gibson, Randy Katz (1988). "A Case for Redundant Arrays of Inexpensive Disks (RAID)". SIGMOD Conference: pp 109–116. retrieved 2006-12-31
This specification suggested a number of prototype RAID levels, or combinations of drives. 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.
Their paper formally defined RAID levels 1 through 5 in sections 7 to 11:
Wikimedia Commons has media related to:
Redundant array of independent disks
There has been a significant amount of research done into the technical aspects of this storage method. Technical institutions and involved companies have released white papers and technical documentation relevant to RAID arrays and made them available to the public. They are accessible below
If you would like more information detailing the deployment, maintenance, and repair of RAID arrays on a specific operating system, the external links below, sorted by operating system, could prove useful.
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia
