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This article is about the computer-related system.

For other uses, see Raid. RAID — which stands for Redundant Array of Inexpensive Disks,or alternatively Redundant Array of Independent Disks (a less specific name and thus now the

RAID — which stands for Redundant Array of Inexpensive Drives (as named by the inventor) or Redundant Array 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. A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device

The phrase "RAID" is an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. An umbrella term is a word that provides a Superset or grouping of related concepts also called a Hypernym. Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital In Computer science, data is anything in a form suitable for use with a Computer. RAID's various designs all involve two key design goals: increased data reliability and increased input/output performance. Data redundancy (sometimes incorrectly referred to as data reliability) in Computer data storage, is a property of some Disk arrays (most commonly in In Computing, input/output, or I/O, refers to the communication between an Information processing system (such as a Computer) and the outside When several physical disks are set up to use RAID technology, they are said to be in a RAID. This array distributes data across several disks, but the array is seen by the computer user and operating system as one single disk. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination RAID can be set up to serve several different purposes, the most common of which are outlined below.

Contents

Purpose and basics

A RAID distributes data across several physical disks which look to the operating system and the user like a single disk. Several different arrangements are possible. We assume here that all the disks are of the same capacity, as is usual.

Some arrays are "redundant" in a way that writes extra data derived from the original data across the array organized so that the failure of one (sometimes more) disks in the array will not result in loss of data; the bad disk is replaced by a new one, and the data on it reconstructed from the remaining data and the extra data. In Engineering, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the System, usually A redundant array obviously allows less data to be stored; a 2-disk RAID 1 array loses half of its capacity, and a RAID 5 array with several disks loses the capacity of one disk.

Other RAIDs are arranged in a way that makes them faster to write to and read from than a single disk.

There are various combinations of these approaches giving different trade offs of protection against data loss, capacity, and speed. RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.

RAID 0 (striped disks) distributes data across several disks in a way which gives improved speed and full capacity, but all data on all disks will be lost if any one disk fails.

RAID 1 (mirrored disks) uses two (possibly more) disks which each store the same data, so that data is not lost so long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure or decrease the reliability of the remaining drives (second, third, etc).

RAID 5 (striped disks with parity) combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk. The less common RAID 6 can recover from the loss of two disks.

RAID involves significant computation when reading and writing information. With true hardware RAID the controller does the work. In other cases the operating system or simpler and less expensive controllers require the host computer's processor to do the computing, which reduces the computer's performance on processor-intensive tasks (see "Software RAID" and "Fake RAID" below). Simpler RAID controllers may provide only levels 0 and 1, which require less processing. A disk array controller is a device which manages the physical Disk drives and presents them to the computer as logical units.

RAID systems with redundancy continue working without interruption when one, or sometimes more, disks of the array fail, although they are vulnerable to further failures. When the bad disk is replaced by a new one the array is rebuilt while the system continues to operate normally. Some systems have to be shut down when removing or adding a drive; others support hot swapping, allowing drives to be replaced without powering down. Hot swapping and hot plugging are terms used to separately describe the functions of replacing system components hot swapping describes changing components like fans and RAID with hot-swap drives is often used in high availability systems, where it is important that the system keeps running as much of the time as possible. High availability is a System design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period

It is important to note that RAID is not an alternative to backing up data. In Information technology, backup refers to making copies of Data so that these additional copies may be used to restore the original after a Data may become damaged or destroyed without harm to the drive(s) on which it is stored. For example, part of the data may be overwritten by a system malfunction; a file may be damaged or deleted by user error or malice and not noticed for days or weeks; and of course the entire array is at risk of catastrophes such as theft, flood, and fire.

RAID principles

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. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination 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). In Data storage, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical Hard disks in real time In Computer data storage, data striping is the segmentation of logically sequential data such as a single file so that segments can be assigned to multiple physical devices In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in In Engineering, Fault-tolerant design, also known as fail-safe design, is a design that enables a system to continue operation possibly at a reduced level (also known 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. Video on demand ( VOD) or Audio video on demand ( AVOD) systems allow users to select and watch/listen

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 disk array is a Disk storage system which contains multiple Disk drives It is differentiated from a Disk enclosure, in that an array has Cache

Standard levels

Main article: Standard RAID levels

A number of standard schemes have evolved which are referred to as levels. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or parity There were five RAID levels originally conceived, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary). See also RAID To gain performance and/or additional redundancy the Standard RAID levels can be combined to create hybrid or Nested RAID levels. See also RAID Although all implementations of RAID differ from the idealized specification to some extent some companies have developed non-standard RAID implementations Proprietary software is Computer software on which the producer has set restrictions on use private modification copying, or republishing.

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/[Non-Redundant Array]. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or parity In Computer data storage, data striping is the segmentation of logically sequential data such as a single file so that segments can be assigned to multiple physical devices Error detection If an odd number of bits (including the parity bit are changed in transmission of a set of bits then parity bit will be incorrect and will thus indicate 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. 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 RAID Level 0
RAID 1 Mirrored set without parity. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or parity In Data storage, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical Hard disks in real time Provides fault tolerance from disk errors and failure of all but one of the drives. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. A thread in Computer science is short for a thread of execution. Array continues to operate so long as at least one drive is functioning. SNIA definition. Using RAID 1 with a separate controller for each disk is sometimes called duplexing. 2 RAID Level 1
RAID 2 Redundancy through Hamming code. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or parity In Telecommunication, a Hamming code is a linear Error-correcting code named after its inventor Richard Hamming. Disks are synchronised and striped in very small stripes, often in single bytes/words. Hamming codes error correction is calculated across corresponding bits on disks, and is stored on multiple parity disks. In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in SNIA definition 3
RAID 3 Striped set with dedicated parity/Bit interleaved parity. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or 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 RAID Level 3
RAID 4 Block level parity. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or parity Identical to RAID 3 but does block-level striping instead of byte-level striping. In this size of each stripe is large i. e. Block. Now a single file can be stored in a block, In this each disk operates independently and many different I/O request can be satisfied in parallel. But data transfer speed for each I/O request will be less. The error detection is done by block level parity and is stored in separate single disk unit. SNIA definition 3 RAID Level 4
RAID 5 Striped set with distributed parity. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or 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 RAID Level 5
RAID 6 Striped set with dual distributed parity. See also RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring or 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 RAID Level 6

Nested levels

Main article: Nested RAID levels

Many storage controllers allow RAID levels to be nested: the elements of a RAID may be either individual disks or RAIDs themselves. See also RAID To gain performance and/or additional redundancy the Standard RAID levels can be combined to create hybrid or Nested RAID levels. Nesting more than two deep is unusual.

As there is no basic RAID level numbered larger than 9, nested RAIDs are usually unambiguously described by concatenating the numbers indicating the RAID levels, sometimes with a "+" in between. For example, RAID 10 (or RAID 1+0) consists of several level 1 arrays of physical drives, each of which is one of the "drives" of a level 0 array striped over the level 1 arrays. RAID 0+1 is not called RAID 01, to avoid confusion with RAID 1. When the top array is a RAID 0 (such as in RAID 10 and RAID 50) most vendors omit the "+", though RAID 5+0 is clearer.

Non-standard levels

Main article: Non-standard RAID levels

Many configurations other than the basic numbered RAID levels are possible, and many companies, organizations, and groups have created their own non-standard configurations, in many cases designed to meet the specialised needs of a small niche group. See also RAID Although all implementations of RAID differ from the idealized specification to some extent some companies have developed non-standard RAID implementations Most of these non-standard RAID levels are proprietary. The word proprietary indicates that a party or proprietor exercises private Ownership, control or use over an item of Property.

Some of the more prominent modifications are:

Implementations

The distribution of data across multiple drives can be managed either by dedicated hardware or by software. Hardware is a general term that refers to the physical artifacts of a Technology. 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.

Operating system based ("software RAID")

Software implementations are now provided by many operating systems. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination A software layer sits above the (generally block-based) disk device drivers and provides an abstraction layer between the logical drives (RAIDs) and physical drives. In computing a device driver or software driver is a Computer program allowing higher-level computer programs to interact with a Hardware device A logical disk is a device that provides an area of usable storage capacity on one or more physical Disk drive components in a computer system Disk storage is a general category of a Computer storage mechanisms in which data is recorded on planar round and rotating surfaces ( disks, discs, or 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) are supported.

Microsoft's server operating systems support 3 RAID levels; RAID 0, RAID 1, and RAID 5. Some of the Microsoft desktop operating systems support RAID such as Windows XP Professional which supports RAID level 0 in addition to spanning multiple disks but only if using dynamic disks and volumes.

Apple's Mac OS X Server supports RAID 0, RAID 1, and RAID 1+0. Mac OS X Server is the server-oriented version of Apple 's Operating system, Mac OS X. [2]

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 specific 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 RAIDs to be created from partitions rather than entire physical drives. Disk partitioning is the creation of separate divisions of a Hard disk drive using Partition editors Once a disk is divided into several partitions directories and For instance, an administrator 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. Intel Matrix RAID is a RAID layout employing two physical Hard disks in a single array in which part of each disk is assigned to a RAID 0 volume and ) Using two partitions on the same drive in the same RAID is, however, dangerous. If, for example, a RAID 5 array is composed of four drives 250 + 250 + 250 + 500 GB, with the 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-based

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. A disk array controller is a device which manages the physical Disk drives and presents them to the computer as logical units. On a desktop system this may be a PCI expansion card, PCI-Express Expansion Card or built into the motherboard. The Peripheral Component Interconnect, or PCI Standard (commonly PCI) specifies a Computer bus for attaching peripheral devices to a Computer An expansion card (also expansion board, adapter card or accessory card) in Computing is a Printed circuit board that can be inserted A motherboard is the central or primary Printed circuit board (PCB making up a complex electronic system such as a modern Computer or Laptop Controllers supporting most types of drive may be used - IDE/ATA, SATA, SCSI, SSA, Fibre Channel, sometimes even a combination. AT Attachment with Packet Interface ( ATA/ATAPI) is a standard interface used to connect storage devices such as Hard disks Solid-state Serial Storage Architecture (SSA is a serial transport protocol used to attach Disk drives to servers It was invented by Ian Judd of IBM in Fibre Channel, or FC, is a Gigabit -speed network technology primarily used for Storage networking. The controller and disks may be in a stand-alone disk enclosure, rather than inside a computer. A disk enclosure is essentially a specialized Chassis designed to hold and power Disk drives while providing a mechanism to allow them to communicate to one or more The enclosure may be directly attached to a computer, or connected via SAN. Direct-attached storage (DAS refers to a digital storage system directly attached to a server or Workstation, without a storage network in between In Information technology, a storage area network ( SAN) is an architecture to attach remote computer storage devices (such as Disk arrays tape libraries 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 Computer science, a cache (kæʃ like "cash") is a collection of data duplicating original In most systems write cache may be non-volatile (i. e. 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. A logical disk is a device that provides an area of usable storage capacity on one or more physical Disk drive components in a computer system

Hardware implementations also typically support hot swapping, allowing failed drives to be replaced while the system is running.

Firmware/driver based RAID ("fake RAID")

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 cheap "RAID controllers" were introduced that do not contain a RAID controller chip, but simply a standard disk controller chip with special firmware and drivers. During early stage bootup the RAID is implemented by the firmware; when a protected-mode operating system such as a modern version of GNU/Linux or Microsoft Windows is loaded the drivers take over. Linux (commonly pronounced ˈlɪnəks

These controllers are described by their manufacturers as RAID controllers, and it is rarely made clear to purchasers that the burden of RAID processing is borne by the host computer's central processing unit, not the RAID controller itself, thus introducing the afore-mentioned CPU overhead. Before their introduction, a "RAID controller" implied that the controller did the processing, and the new type has become known in technically knowledgeable circles as "fake RAID" even though the RAID itself is implemented correctly.

Network-Attached Storage

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. Network-attached storage ( NAS) is file-level Computer data storage connected to a Computer network providing data access to heterogeneous network clients A computer network is a group of interconnected Computers. Networks may be classified according to a wide variety of characteristics Ethernet is a family of frame -based Computer networking technologies for Local area networks (LANs 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.

Hot spares

Both hardware and software RAIDs with redundancy may support the use of hot spare drives, a drive physically installed in the array which is inactive until an active drive fails, when the system automatically replaces the failed drive with the spare, rebuilding the array with the spare drive included. A hot spare or hot standby is used as a Failover mechanism to provide reliability in system configurations This reduces the mean time to repair (MTTR), though it doesn't eliminate it completely. Mean time to recovery(MTTR is the average time that a device will take to recover from any failure A second drive failure in the same RAID redundancy group before the array is fully rebuilt will result in loss of the data; rebuilding can take several hours, especially on busy systems.

Rapid replacement of failed drives is important as the drives of an array will all have had the same amount of use, and may tend to fail at about the same time rather than randomly. RAID 6 without a spare uses the same number of drives as RAID 5 with a hot spare and protects data against simultaneous failure of up to two drives, but requires a more advanced RAID controller.

Reliability terms

Failure rate
The mean time to failure (MTTF) or the mean time between failure (MTBF) of a given RAID is the same as those of its constituent hard drives, regardless of what type of RAID is employed. Failure rate is the Frequency with which an engineered system or component fails, expressed for example in failures per Hour.
Mean time to data loss (MTTDL)
In this context, the average time before a loss of data in a given array. [3]. Mean time to data loss of a given RAID may be higher or lower than that of its constituent hard drives, depending upon what type of RAID is employed.
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. Mean time to recovery(MTTR is the average time that a device will take to recover from any failure 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.
Write cache reliability
Some RAID systems use RAM write cache to increase performance. Failure of the RAM can lose data.
Atomic write failure
Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, non-transactional, etc.

Issues with RAID

Correlated failures

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. [4]

Atomicity

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" [5]during the early days of relational database commercialization. James Nicholas "Jim" Gray (born 1944 lost at sea January 28, 2007) was an American Computer scientist who received the Turing 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. NTFS (New Technology File System Is the standard File system of Windows NT, including its later versions Windows 2000, Windows XP, Windows NetApp WAFL file system solves it by never updating the data in place, as does ZFS. The Write Anywhere File Layout ( WAFL) is a File system that supports large high-performance RAID arrays quick restarts without lengthy consistency checks In Computing, ZFS is a File system designed by Sun Microsystems for the Solaris Operating System.

Unrecoverable data

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. A bad sector is a sector on a computer's Disk drive that cannot be used due to permanent damage such as physical damage to the disk particles 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/PATA, SATA). Fibre Channel, or FC, is a Gigabit -speed network technology primarily used for Storage networking. Introduction A typical Serial Attached SCSI system consists of the following basic components An Initiator is a device that originates device service 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.

Write cache reliability

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.

Equipment compatibility

The disk formats on different RAID controllers are not necessarily compatible, so that it may not be possible to read a RAID on different hardware. Consequently a non-disk hardware failure may require using identical hardware, or a data backup, to recover the data.

History

Norman Ken Ouchi at IBM was awarded a 1978 U.S. patent 4,092,732[6] titled "System for recovering data stored in failed memory unit. International Business Machines Corporation abbreviated IBM and nicknamed "Big Blue", is a multinational Computer Technology United States patent law was established "to promote the Progress of Science and useful Arts by securing for limited Times to Authors and Inventors the exclusive Right to their respective " The claims for this patent describe what would later be termed RAID 5 with full stripe writes. Patent claims are usually in the form of a series of specified elements and corresponding limitations or more precisely Noun phrases following the description portion of the In Computer data storage, data striping is the segmentation of logically sequential data such as a single file so that segments can be assigned to multiple physical devices 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. Prior art (also known as or State of the art, which also has other meanings in most systems of Patent law constitutes all Information that

The term RAID was first defined by David A. Patterson, Garth A. Gibson and Randy Katz at the University of California, Berkeley in 1987. David Andrew Patterson (born November 16[[ 947]] is an American computer pioneer and academic who has held the position of Professor of Computer Science Garth Gibson is a Computer Scientist from Carnegie Mellon University. Dr Randy Howard Katz is a Distinguished professor at University of California Berkeley of the Electrical Engineering The University of California Berkeley (also referred to as Cal, Berkeley and UC Berkeley) is a major research university located in Berkeley 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. SIGMOD is the Association for Computing Machinery 's Special Interest Group on Management of Data which specializes in large-scale data management problems and Databases [7]

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:

See also

References

  1. ^ Main Page - Linux-raid
  2. ^ Apple Mac OS X Server File Systems. In Telecommunication, a Hamming code is a linear Error-correcting code named after its inventor Richard Hamming. In Mathematics, Computer science, Telecommunication, and Information theory, error detection and correction has great practical importance in The SNIA Common RAID Disk Data Format defines a standard data structure describing how data is formatted across disks in a RAID group Redundant/Reliable Array of Inexpensive/Independent Nodes ( RAIN) is an Open architecture approach to Computer storage, that combines commodity or low-cost Vinum is a logical volume manager, also called Software RAID, allowing implementations of the RAID-0, RAID-1 and RAID-5 models both individually Retrieved on 2008-04-23. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 215 BC - A temple is built on the Capitoline Hill dedicated to Venus Erycina to commemorate the Roman defeat at
  3. ^ Jim Gray and Catharine van Ingen, "Empirical Measurements of Disk Failure Rates and Error Rates", MSTR-2005-166, December 2005
  4. ^ Disk Failures in the Real World: What Does an MTTF of 1,000,000 Hours Mean to You? Bianca Schroeder and Garth A. Gibson
  5. ^ Jim Gray: The Transaction Concept: Virtues and Limitations (Invited Paper) VLDB 1981: 144-154
  6. ^ U.S. Patent 4,092,732 
  7. ^ Patterson, David; Garth A. Gibson, Randy Katz (1988). Garth Gibson is a Computer Scientist from Carnegie Mellon University. David Andrew Patterson (born November 16[[ 947]] is an American computer pioneer and academic who has held the position of Professor of Computer Science Garth Gibson is a Computer Scientist from Carnegie Mellon University. Dr Randy Howard Katz is a Distinguished professor at University of California Berkeley of the Electrical Engineering "A Case for Redundant Arrays of Inexpensive Disks (RAID)". SIGMOD Conference: pp 109–116. SIGMOD is the Association for Computing Machinery 's Special Interest Group on Management of Data which specializes in large-scale data management problems and Databases   retrieved 2006-12-31

Further reading

External links

White papers

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 RAIDs and made them available to the public. A white paper is an authoritative report or guide that often addresses problems and how to solve them They are accessible below

Operating system-specific details

If you would like more information detailing the deployment, maintenance, and repair of RAIDs on a specific operating system, the external links below, sorted by operating system, could prove useful.

OpenBSD is a Unix-like computer Operating system descended from Berkeley Software Distribution (BSD a Unix derivative developed at the OpenSolaris is an Open source project created by Sun Microsystems to build a developer community around Solaris Operating System technology Mac OS X (mæk oʊ ɛs tɛn is a line of computer Operating systems developed marketed and sold by Apple Inc, the latest of which is pre-loaded on all currently Microsoft Windows is a series of Software Operating systems and Graphical user interfaces produced by Microsoft.
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