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(Redirected from Redundant array of independent disks)
In computing, specifically computer storage, a 'Redundant Array of Independent Drives' (or 'Disks'), also known as 'Redundant Array of Inexpensive Drives' (or 'Disks'), ('RAID') is an umbrella term for data storage schemes that divide and/or replicate data among multiple hard drives. RAID can be designed to provide increased data reliability or increased I/O performance, or both.
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).
RAID combines physical hard disks into a single logical unit either by using 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 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 it insists 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 off 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.
RAID systems can be designed to keep working when there is failure - disks can be hot swapped and data recovered automatically 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 is traditionally used on servers, but can be also used on workstations.
The latter is especially true in storage-intensive computers such as those used for video and audio editing.
Norman Ken Ouchi at IBM was awarded a 1978 U.S. patent 4,092,732[1] 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.[2]
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:
★ First level RAID: mirrored drives
★ Second level RAID: Hamming code for error correction
★ Third level RAID: single check disk per group
★ Fourth level RAID: independent reads and writes
★ Fifth level RAID: spread data/parity over all drives (no single check disk)
RAID levels have been recently standardized by SNIA.[3]
Main articles: Standard RAID levels
A quick summary of the most commonly used RAID levels:
★ 'RAID 0:' striped set (minimum 2 disks) 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. The reason a single disk failure destroys the entire array is 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.
★ 'RAID 1:' mirrored set (minimum 2 disks) 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.
★ 'RAID 3' and 'RAID 4': striped set (minimum 3 disks) with dedicated parity, the parity bits represent a memory location each, they have a value of 0 or 1, whether the given memory location they represent, is empty or full, thus enhancing the speed of read and write. 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 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.
★ 'RAID 5:' striped set (minimum 3 disks) with distributed parity. Distributed parity requires all but one drive 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.
★ 'RAID 6:' striped set (minimum 4 disks) with dual distributed 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. As drives grow in size, they become more prone to error and exposure to failure during fixing, a single drive may be 1 Terabyte in size. 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. With dual parity, it gives time to rebuild the array by recreating a failed drive with the ability to sustain failure on another drive in the same array.
Main articles: 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 drives. It is instructive to think of these arrays as layered on top of each other, with physical drives 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 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 more informative.
★ RAID 0+1: striped sets in a mirrored set (minimum 4 disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if two or more drives fail on different sides of the mirroring, the data on the RAID system is lost.
★ RAID 1+0: mirrored sets in a striped set (minimum 4 disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses as long as no two drives lost comprise a single pair of one mirror.
★ RAID 5+0: stripe across distributed parity RAID systems
★ RAID 5+1: mirror striped set with distributed parity (some manufacturers label this as RAID 53)
Main articles: Non-standard RAID levels
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 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:
★ ATTO Technology's ''DVRAID''™ adds parity RAID protection to systems which demand performance for 4K film, 2K film, high-definition audio and video.
★ Storage Computer Corporation uses ''RAID 7'', which adds caching to RAID 3 and RAID 4 to improve performance.
★ EMC Corporation offered ''RAID S'' as an alternative to RAID 5 on their Symmetrix systems (no longer supported on the latest releases of Enginuity, the Symmetrix's operating system).
★ Solaris and OpenSolaris offer ''RAID-Z'' with the zfs filesystem, that solves the write hole problem of RAID 5.
The distribution of data across multiple drives can be managed either by dedicated hardware or by software. Additionally, there are hybrid RAIDs that are partially software ''and'' hardware-based solutions.
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).
Since the software must run on a host server attached to storage, the processor (as mentioned above) on that host must dedicate processing time to run the RAID software. Like hardware-based RAID, if the server experiences a hardware failure, the attached storage could be inaccessible for a period of time.
Software implementations, especially LVM-like, can allow RAID arrays to be created from partitions rather than entire physical drives. For instance, Novell NetWare allows you to 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 is only really helpful for increasing performance. Especially when drive sizes are unequal, it actually impairs reliability of a system. 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.
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. Any drives may be used - IDE/ATA, SATA, SCSI, SSA, Fibre Channel, sometimes even a combination thereof. In a large environment the controller and disks may be placed outside of a physical machine, in a stand alone disk enclosure. The using machine can be directly attached to the enclosure in a traditional way, 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.
Hybrid RAID implementations have become very popular with the introduction of inexpensive ''RAID controllers'', implemented using a standard disk controller and then implementing the RAID in the controllers BIOS extension (for early boot-up/real mode operation) and the operating system driver (for after the system switches to protected mode). Since these controllers use proprietary disk layouts, they typically cannot span controllers from multiple manufacturers. The only advantages over software RAID are that the BIOS can boot from them, and that tighter integration with the device driver may offer better error handling.
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. 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. It also prevents data loss when multiple drives fail in a short period of time, as is common when all drives in an array have undergone very similar use patterns, and experience wear-out failures.
; 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 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: Some RAID systems use RAM write cache to increase performance. 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 could 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.
; Atomic write failure: Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, non-transactional, etc. 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.
: The battery-backed write cache may partially solve the problem, it is however 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.
★ RAID controller
★ Disk array
★ Vinum volume manager
★ Storage area network (SAN)
★ Hard disk
★ Redundant Array of Inexpensive Nodes
★ Redundant Array of Independent Filesystems
★ Redundant Array of Inexpensive Servers
★ Disk Data Format (DDF)
1.
2.
retrieved 2006-12-31
3. Common RAID Disk Data Format Specification retrieved 2007-06-25
★ RAID Disk Space Calculator
★ Working RAID illustrations
★ RAID Levels — Tutorial and Diagrams
★ Slashdot: Which RAID for a Personal Fileserver? — Comments from network specialists and enthusiasts.
★ Logical Volume Manager Performance Measurement
★ Animations and Descriptions to help Learn about RAID Levels 0, 1, 5, 10, and 50
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
★ Tutorial on Reed-Solomon Coding for Fault-Tolerance in RAID-like Systems
★ Parity Declustering for Continuous Operation in Redundant Disk Arrays
★ An Optimal Scheme for Tolerating Double Disk Failures in RAID Architectures
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.
★ IBM - seriesI/iSeries/AS400 Running OS400/i5OS
★
★ IBM Tech reference
★ Novell
★
★ Novell NetWare TTS transactional tracking system
★ Linux
★
★ Linux Software-RAID HOWTO
★
★ RAID-1 QUICK Howto under Linux (Note: Site appears to be down as of 5/21/2007).
★
★ Experiences w/ Software RAID 5 Under Linux? — "Ask Slashdot" article on RAID 5.
★
★ Optimal Hardware RAID Configuration for Linux. Make it up to 30% faster by aligning the file system to the RAID stripe structure.
★ OpenSolaris
★
★ ZFS in action: 100 Mirrored filesystems in 5 minutes
★ Mac OS X
★
★ Creating a RAID set
★
★ Selecting options for your RAID set
★
★ Using several drives as a single volume with a concatenated RAID set
★
★ Also see "man diskutil" at a command line prompt in OS X's Terminal.app, or read it here
★ Microsoft Windows
★
★ Basic Storage Versus Dynamic Storage in Windows XP RAID functionality built into Windows 2000/XP
★
★ Windows Software RAID Guide
In computing, specifically computer storage, a 'Redundant Array of Independent Drives' (or 'Disks'), also known as 'Redundant Array of Inexpensive Drives' (or 'Disks'), ('RAID') is an umbrella term for data storage schemes that divide and/or replicate data among multiple hard drives. RAID can be designed to provide increased data reliability or increased I/O performance, or both.
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).
Overview
RAID combines physical hard disks into a single logical unit either by using 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 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 it insists 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 off 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.
RAID systems can be designed to keep working when there is failure - disks can be hot swapped and data recovered automatically 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 is traditionally used on servers, but can be also used on workstations.
The latter is especially true in storage-intensive computers such as those used for video and audio editing.
History
Norman Ken Ouchi at IBM was awarded a 1978 U.S. patent 4,092,732[1] 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.[2]
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:
★ First level RAID: mirrored drives
★ Second level RAID: Hamming code for error correction
★ Third level RAID: single check disk per group
★ Fourth level RAID: independent reads and writes
★ Fifth level RAID: spread data/parity over all drives (no single check disk)
RAID levels have been recently standardized by SNIA.[3]
Standard RAID levels
Main articles: Standard RAID levels
A quick summary of the most commonly used RAID levels:
RAID Level 0
★ 'RAID 0:' striped set (minimum 2 disks) 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. The reason a single disk failure destroys the entire array is 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.
★ 'RAID 1:' mirrored set (minimum 2 disks) 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.
★ 'RAID 3' and 'RAID 4': striped set (minimum 3 disks) with dedicated parity, the parity bits represent a memory location each, they have a value of 0 or 1, whether the given memory location they represent, is empty or full, thus enhancing the speed of read and write. 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 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.
★ 'RAID 5:' striped set (minimum 3 disks) with distributed parity. Distributed parity requires all but one drive 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.
★ 'RAID 6:' striped set (minimum 4 disks) with dual distributed 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. As drives grow in size, they become more prone to error and exposure to failure during fixing, a single drive may be 1 Terabyte in size. 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. With dual parity, it gives time to rebuild the array by recreating a failed drive with the ability to sustain failure on another drive in the same array.
Nested RAID levels
Main articles: 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 drives. It is instructive to think of these arrays as layered on top of each other, with physical drives 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 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 more informative.
Common nested RAID levels
★ RAID 0+1: striped sets in a mirrored set (minimum 4 disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if two or more drives fail on different sides of the mirroring, the data on the RAID system is lost.
★ RAID 1+0: mirrored sets in a striped set (minimum 4 disks; even number of disks) provides fault tolerance and improved performance but increases complexity. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. The array can sustain multiple drive losses as long as no two drives lost comprise a single pair of one mirror.
★ RAID 5+0: stripe across distributed parity RAID systems
★ RAID 5+1: mirror striped set with distributed parity (some manufacturers label this as RAID 53)
Non-standard RAID levels
Main articles: Non-standard RAID levels
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 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:
★ ATTO Technology's ''DVRAID''™ adds parity RAID protection to systems which demand performance for 4K film, 2K film, high-definition audio and video.
★ Storage Computer Corporation uses ''RAID 7'', which adds caching to RAID 3 and RAID 4 to improve performance.
★ EMC Corporation offered ''RAID S'' as an alternative to RAID 5 on their Symmetrix systems (no longer supported on the latest releases of Enginuity, the Symmetrix's operating system).
★ Solaris and OpenSolaris offer ''RAID-Z'' with the zfs filesystem, that solves the write hole problem of RAID 5.
RAID implementations
Hardware and/or software RAID based solutions
The distribution of data across multiple drives can be managed either by dedicated hardware or by software. Additionally, there are hybrid RAIDs that are partially software ''and'' hardware-based solutions.
Software RAID
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).
Since the software must run on a host server attached to storage, the processor (as mentioned above) on that host must dedicate processing time to run the RAID software. Like hardware-based RAID, if the server experiences a hardware failure, the attached storage could be inaccessible for a period of time.
Software implementations, especially LVM-like, can allow RAID arrays to be created from partitions rather than entire physical drives. For instance, Novell NetWare allows you to 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 is only really helpful for increasing performance. Especially when drive sizes are unequal, it actually impairs reliability of a system. 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
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. Any drives may be used - IDE/ATA, SATA, SCSI, SSA, Fibre Channel, sometimes even a combination thereof. In a large environment the controller and disks may be placed outside of a physical machine, in a stand alone disk enclosure. The using machine can be directly attached to the enclosure in a traditional way, 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.
Hybrid RAID
Hybrid RAID implementations have become very popular with the introduction of inexpensive ''RAID controllers'', implemented using a standard disk controller and then implementing the RAID in the controllers BIOS extension (for early boot-up/real mode operation) and the operating system driver (for after the system switches to protected mode). Since these controllers use proprietary disk layouts, they typically cannot span controllers from multiple manufacturers. The only advantages over software RAID are that the BIOS can boot from them, and that tighter integration with the device driver may offer better error handling.
Hot spares
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. 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. It also prevents data loss when multiple drives fail in a short period of time, as is common when all drives in an array have undergone very similar use patterns, and experience wear-out failures.
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 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: Some RAID systems use RAM write cache to increase performance. 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 could 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.
; Atomic write failure: Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, non-transactional, etc. 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.
: The battery-backed write cache may partially solve the problem, it is however 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.
See also
★ RAID controller
★ Disk array
★ Vinum volume manager
★ Storage area network (SAN)
★ Hard disk
★ Redundant Array of Inexpensive Nodes
★ Redundant Array of Independent Filesystems
★ Redundant Array of Inexpensive Servers
★ Disk Data Format (DDF)
References
1.
2.
retrieved 2006-12-31
3. Common RAID Disk Data Format Specification retrieved 2007-06-25
External links
★ RAID Disk Space Calculator
★ Working RAID illustrations
★ RAID Levels — Tutorial and Diagrams
★ Slashdot: Which RAID for a Personal Fileserver? — Comments from network specialists and enthusiasts.
★ Logical Volume Manager Performance Measurement
★ Animations and Descriptions to help Learn about RAID Levels 0, 1, 5, 10, and 50
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 RAID arrays and made them available to the public. They are accessible below
★ Tutorial on Reed-Solomon Coding for Fault-Tolerance in RAID-like Systems
★ Parity Declustering for Continuous Operation in Redundant Disk Arrays
★ An Optimal Scheme for Tolerating Double Disk Failures in RAID Architectures
Operating system-specific details
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.
★ IBM - seriesI/iSeries/AS400 Running OS400/i5OS
★
★ IBM Tech reference
★ Novell
★
★ Novell NetWare TTS transactional tracking system
★ Linux
★
★ Linux Software-RAID HOWTO
★
★ RAID-1 QUICK Howto under Linux (Note: Site appears to be down as of 5/21/2007).
★
★ Experiences w/ Software RAID 5 Under Linux? — "Ask Slashdot" article on RAID 5.
★
★ Optimal Hardware RAID Configuration for Linux. Make it up to 30% faster by aligning the file system to the RAID stripe structure.
★ OpenSolaris
★
★ ZFS in action: 100 Mirrored filesystems in 5 minutes
★ Mac OS X
★
★ Creating a RAID set
★
★ Selecting options for your RAID set
★
★ Using several drives as a single volume with a concatenated RAID set
★
★ Also see "man diskutil" at a command line prompt in OS X's Terminal.app, or read it here
★ Microsoft Windows
★
★ Basic Storage Versus Dynamic Storage in Windows XP RAID functionality built into Windows 2000/XP
★
★ Windows Software RAID Guide
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