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RAID LEVELS Basic functions At the very simplest level, RAID combines multiple hard drives into a single logical unit. Thus, instead of seeing several different hard drives, the operating system sees only one. RAID is typically used on server computers, and is usually (but not necessarily) implemented with identically sized disk drives. With decreases in hard drive prices and wider availability of RAID options built into motherboard chipsets, RAID is also being found and offered as an option in more advanced personal computers. This is especially true in computers dedicated to storage-intensive tasks, such as video and audio editing. History Norman Ken Ouchi at IBM was awarded a 1978 US patent 4,092.732 [1] titled "System for recovering data stored in failed memory unit" and the claims for this patent describe what would later be termed RAID 5 with full stripe writes. This 1978 patent also mentions that disk mirroring or duplexing (what would later be termed RAID 1) and protection with dedicated parity (that would later be termed RAID 4) were prior art at that time. The original RAID specification suggested a number of prototype "RAID levels", or combinations of disks. Each had theoretical advantages and disadvantages. Over the years, different implementations of the RAID concept have appeared. Most differ substantially from the original idealized RAID levels, but the numbered names have remained. This can be confusing, since one implementation of RAID 5, for example, can differ substantially from another. RAID 3 and RAID 4 are often confused and even used interchangeably. RAID technology 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 disks 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] Their paper formally defined RAID levels 1 through 5:

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section 7: "First Level RAID: Mirrored Disks" section 8: "Second Level RAID: Hamming Code for Error Correction" section 9: "Third Level RAID: Single Check Disk Per Group" section 10: "Fourth Level RAID: Independent Reads and Writes" section 11: "Fifth Level RAID: Spread data/parity over all disks (no single check disk)"

Their paper is also the origination of the term "RAID", to which the second meaning "Independent" for the "I" has been since used. [3]

The very definition of RAID has been argued over the years. The use of the term redundant leads many to object to RAID 0 being called a RAID at all. [citation needed] Similarly, the change from inexpensive to independent confuses many as to the intended purpose of RAID. [citation needed] There are even some single-disk implementations of the RAID concept. [citation needed] For the purpose of this article, it is best to assume that any system which employs the basic RAID concepts to combine physical disk space for purposes of reliability, capacity, or performance is a RAID system. RAID implementations Hardware vs. software The distribution of data across multiple disks can be managed by either dedicated hardware or by software. Additionally, there are hybrid RAIDs that are partially software AND hardware-based solutions. With a software implementation, the operating system manages the disks of the array through the normal drive controller (IDE/ATA, SATA, SCSI, Fibre Channel, etc.). With present CPU speeds, software RAID can be faster than hardware RAID[4], though at the cost of using CPU power which might be best used for other tasks. One major exception is where the hardware implementation of RAID incorporates a battery backed-up write back cache which can speed up an application, such as an OLTP database server. In this case, the hardware RAID implementation flushes the write cache to secure storage to preserve data at a known point if there is a crash. The hardware approach is faster than accessing the disk drive and limited by RAM speeds, the rate at which the cache can be mirrored to another controller, the amount of cache and how fast it can flush the cache to disk. For this reason, battery-backed caching disk controllers are often recommended for high transaction rate database servers. In the same situation, the software solution is limited to no more flushes than the number of rotations or seeks per second of the drives. Another disadvantage of a pure software RAID is that, depending on the disk that fails and the boot arrangements in use, the computer may not be able to be rebooted until the array has been rebuilt.

A hardware implementation of RAID requires at a minimum a special-purpose RAID controller. On a desktop system, this may be a PCI expansion card, or might be a capability built in to the motherboard. In larger RAIDs, the controller and disks are usually housed in an external multi-bay enclosure. The disks may be IDE/ATA, SATA, SCSI, Fibre Channel, or any combination thereof. The controller links to the host computer(s) with one or more high-speed SCSI, PCIe, Fibre Channel or iSCSI connections, either directly, or through a fabric, or is accessed as network-attached storage. This controller handles the management of the disks, and performs parity calculations (needed for many RAID levels). This option tends to provide better performance, and makes operating system support easier. Hardware implementations also typically support hot swapping, allowing failed drives to be replaced while the system is running. In rare cases hardware controllers have become faulty, which can result in data loss. Hybrid RAIDs have become very popular with the introduction of inexpensive hardware RAID controllers. The hardware is a normal disk controller that has no RAID features, but there is a boot-time application that allows users to set up RAIDs that are controlled via the BIOS. When any modern operating system is used, it will need specialized RAID drivers that will make the array look like a single block device. Since these controllers actually do all calculations in software, not hardware, they are often called "fakeraids". Unlike software RAID, these "fakeraids" typically cannot span multiple controllers. Both hardware and software versions may support the use of a hot spare, a preinstalled drive which is used to immediately (and almost always automatically) replace a drive that has failed. This reduces the mean time to repair period during which a second drive failure in the same RAID redundancy group can result in loss of data. Some software RAID systems allow one to build arrays from partitions instead of whole disks. Unlike Matrix RAID they are not limited to just RAID 0 and RAID 1 and not all partitions need to be RAID. Standard RAID levels Main article: Standard RAID levels A quick summary of the most commonly used RAID levels:

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RAID 0: Striped Set RAID 1: Mirrored Set RAID 5: Striped Set with Distributed Parity

Common nested RAID levels:

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RAID 01: A mirror of stripes RAID 10: A stripe of mirrors RAID 50: A stripe across dedicated parity RAID systems RAID 100: A stripe of a stripe of mirrors

Nested RAID levels Main article: Nested RAID levels Many storage controllers allow RAID levels to be nested. That is, one RAID can use another as its basic element, instead of using physical disks. It is instructive to think of these arrays as layered on top of each other, with physical disks at the bottom. Nested RAIDs are usually signified by joining the numbers indicating the RAID levels into a single number, sometimes with a '+' in between. For example, RAID 10 (or RAID 1+0) conceptually consists of multiple level 1 arrays stored on physical disks with a level 0 array on top, striped over the level 1 arrays. In the case of RAID 0+1, it is most often called RAID 0+1 as opposed to RAID 01 to avoid confusion with RAID 1. However, when the top array is a RAID 0 (such as in RAID 10 and RAID 50), most vendors choose to omit the '+', though RAID 5+0 is more informative. Non-standard RAID levels Main article: 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 RAID protection to systems delivering high-definition audio and video

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The Storage Computer Corporation uses RAID 7 add caching to RAID 3 and RAID 4 to improve performance EMC Corporation offer's RAID S as an alternative to RAID 5 on their Symmetrix systems. RAID-Z in the zfs filesystem of OpenSolaris solves the "write hole" problem of RAID-5.

What RAID Can and Cannot Do This guide was taken from a thread in a RAID-related forum to help clarify the advantages and disadvantages to choosing RAID for either increases in performance or redundancy. It contains links to other threads in its forum containing user-generated anecdotal reviews of their RAID experiences. What RAID Can Do

RAID can protect uptime. RAID levels 1, 0+1/10, 5, and 6 (and their variants such as 50 and 51) allow a mechanical hard disk to fail while keeping the data on the array accessible to users. Rather than being required to perform a time consuming restore from tape, DVD, or other slow backup media, RAID allows data to be restored to a replacement disk from the other members of the array, while being simultaneously available to users in a degraded state. This is of high value to enterprises, as downtime quickly leads to lost earning power. For home users, it can protect uptime of large media storage arrays, which would require time consuming restoration from dozens of DVD or quite a few tapes in the event of a disk failing that is not protected by redundancy.

RAID can increase performance in certain applications. RAID levels 0, and 5-6 all use variations on striping, which allows multiple spindles to increase sustained transfer rates when conducting linear transfers. Workstation type applications that work with large files, such as image and video editing applications, benefit greatly from disk striping. The extra throughput offered by disk striping is also useful in disk-to-disk backups applications. Also if RAID 1 or a striping based RAID with a sufficiently large block size is used RAID can provide performance improvements for access patterns involving multiple simultaneous random accesses (e.g., multi-user databases).

What RAID Cannot Do

RAID cannot protect the data on the array. A RAID array has one file system. This creates a single point of failure. A RAID array's file system is vulnerable to a wide variety of hazards other than physical disk failure, so RAID cannot defend against these sources of data loss. RAID will not stop a virus from destroying data. RAID will not prevent corruption. RAID will not save data from accidental modification or deletion by the user. RAID does not protect data from hardware failure of any component besides physical disks. RAID does not protect data from natural or man made disaster such as fires and floods. To protect data, data must be backed up to removable media, such as DVD, tape, or an external hard drive, and stored in an off site location. RAID alone will not prevent a disaster from turning into data loss. Disaster is not preventable, but backups allow data loss to be prevented.

RAID cannot simplify disaster recovery*. When running a single disk, the disk is usually accessible with a generic ATA or SCSI driver built into most operating systems. However, most RAID controllers require specific drivers. Recovery tools that work with single disks on generic controllers will require special drivers to access data on RAID arrays. If these recovery tools are poorly coded and do not allow providing for additional drivers, then a RAID array will probably be inaccessible to that recovery tool.

RAID cannot provide a performance boost in all applications. This statement is especially true with typical desktop application users and gamers. Most desktop applications and games place performance emphasis on the buffer strategy and seek performance of the disk(s). Increasing raw sustained transfer rate shows little gains for desktop users and gamers, as most files that they access are typically very small anyway. Disk striping using RAID 0 increases linear transfer performance, not buffer and seek performance. As a result, disk striping using RAID 0 shows little to no performance gain in most desktop applications and games, although there are exceptions. For desktop users and gamers with high performance as a goal, it is better to buy a faster, bigger, and more expensive single disk than it is to run two slower/smaller drives in RAID 0. Even running the large high quality drive in RAID-0 is unlikely to boost performance more than 10% and performance may drop in some access patterns, particularly games.

RAID is not readily moved to a new system*. When using a single disk, it is relatively straightforward to move the disk to a new system. Simply connect it to the new system, provided it has the same interface available. However, this is not so easy with a RAID array. A RAID BIOS must be able to read metadata from the array members in order to successfully construct the array and make it accessible to an operating system. Since RAID controller makers use different formats for their metadata (even controllers of different families from the same manufacturer may use incompatible metadata formats) it is virtually impossible to move a RAID array to a different controller. When moving a RAID array to a new system, plans should be made to move the controller as well. With the popularity of motherboard integrated RAID controllers, this is extremely difficult to accomplish. Generally, it is possible to move the RAID array members and controllers as a unit, and software RAID in Linux and Windows Server Products can also work around this limitation, but software RAID has other limitations (mostly performance related).

* RAID level 1 does partly circumvent these problems, since each of the disks in the array theoretically contains exactly the same data as a single disk would, if RAID wasn't used.

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 failuretolerant mode of operation. This includes time to replace a failed disk mechanism as well as time to re-build the array (i.e. to replicate data for redundancy). Unrecoverable bit error rate (UBE) This is the rate at which a disk drive will be unable to recover data after application of cyclic redundancy check (CRC) codes and multiple retries. This failure will present as a sector read failure. Some RAID implementations protect against this failure mode by remapping the bad sector, using the redundant data to retrieve a good copy of the data, and rewriting that good data to the newly mapped replacement sector. The UBE rate is typically specified at 1 bit in 1015 for enterprise class disk drives (SCSI, FC, SAS) , and 1 bit in 1014 for desktop class disk drives (IDE, ATA, SATA). Increasing disk capacities and large RAID 5 redundancy groups have led to an increasing inability to successfully rebuild a RAID group after a disk failure because an unrecoverable sector is found on the remaining disks. Double protection schemes such as RAID 6 are attempting to address this issue, but suffer from a very high write penalty. Atomic Write Failure Also known by various terms such as torn writes, torn pages, incomplete writes, interrupted writes, 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 disks. 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. 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.

Standard RAID levels From Wikipedia, the free encyclopedia Jump to: navigation, search Main article: RAID The standard RAID levels are a basic set of RAID configurations and employ striping, mirroring, or parity. The standard RAID levels can be nested for other benefits (see Nested RAID levels). Contents

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1 Error-correction codes

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3 Concatenation (JBOD)

2 RAID 0 o 2.1 RAID 0 failure rate o 2.2 RAID 0 performance 4 RAID 1 o 4.1 RAID 1 failure rate o 4.2 RAID 1 performance

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5 RAID 2 6 RAID 3 7 RAID 4 8 RAID 5 o 8.1 o 8.2 o 8.3 o 8.4


5 5 5 5

parity handling disk failure rate performance usable size

9 RAID 6 o 9.1 RAID 6 performance o 9.2 RAID 6 implementation

10 RAID 5E, RAID 5EE and RAID 6E

11 See also

Error-correction codes For RAID 2 through 5 an error-correcting code is used to provide redundancy of the data. For RAID 2, a Hamming code is used. For this level, extra disks are needed to store the error-correcting bits ("check disks" according to Patterson, et. al.). The remaining levels use standard parity bits by using the XOR logical function. For example, given the following three bytes:

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A1 = 00000111 A2 = 00000101 A3 = 00000000

Taking the XOR of all of these yields:

= 00000010 In terms of parity, the parity byte creates even parity. This means that the sum of 1s for each bit position yields an even number. In this example, the 2nd bit position from the right has one 1 while 1st and 3rd have two 1s; including the parity yields two (even) parity for these three bit positions. The advantage of using parity is when one disk is lost (primary interest is when it is lost due to hardware failure). For example, say the disk containing A2 is lost leaving A1, A3, and Ap to reconstruct A2. This can be done by using the XOR operation again:

= 00000101 This value clearly matches the above definition of A2. This process can then be repeated for the remainder of the data. All of the above examples use only three data bytes and one parity byte. (This is called a "stripe" in the remainder of this article and are given the same color in the diagrams.) When used in conjunction with RAID these operations happen on blocks (the fundamental operational/usable unit of storage in computer storage) instead of individual bytes. RAID 0

Diagram of a RAID 0 setup. A RAID 0 (also known as a stripe set or striped volume) splits data evenly across two or more disks (striped) with no parity information for redundancy. It is important to note that RAID 0 was not one of the original RAID levels and provides zero redundancy. RAID 0 is normally used to increase performance, although it can also be used as a way to create a small number of large virtual disks out of a large number of small physical ones. A RAID 0 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk. For example, if a 120 GB disk is striped together with a 100 GB disk, the size of the array will be

= 2 * 100GB = 200GB In the diagram to the right, the odd blocks are written to disk 0 while the even blocks are written to disk 1 such that A1, A2, A3, A4, ... would be order of blocks read if read sequentially from the beginning. RAID 0 failure rate Although RAID 0 was not specified in the original RAID paper, an idealized implementation of RAID 0 would split I/O operations into equal-sized blocks and spread them evenly across two disks. RAID 0 implementations with more than two disks are also possible, however the group reliability decreases with member size. Reliability of a given RAID 0 set is equal to the average reliability of each disk divided by the number of disks in the set:

That is, reliability (as measured by mean time to failure (MTTF) or mean time between failures (MTBF) is roughly inversely proportional to the number of members — so a set of two disks is roughly half as reliable as a single disk (in other words, the probability of a failure is roughly proportional to the number of members. If there were a probability of 5% that the disk would die within three years, in a two disk array, that probability would be upped to 1 − (1 − 0.05)2 = 0.0975 = 9.75%). The reason for this is that the file system is distributed across all disks. When a drive fails the file system cannot cope with such a large loss of data and coherency since the data is "striped" across all drives (the data cannot be recovered with the missing disk). Data can be recovered using special tools (see data recovery), however, this data will be incomplete and most likely corrupt, and recovery of drive data is very costly nor guaranteed. RAID 0 performance While the block size can technically be as small as a byte it is almost always a multiple of the hard disk sector size of 512 bytes. This lets each drive seek independently when randomly reading or writing data on the disk. How much the drives act independently depends on the access pattern from the filesystem level. For reads and writes that are larger than the stripe size such as copying files or video playback the disks will be seeking to the same position on each disk so the seek time of the array will be the same as that of a single non raid drive. For reads and writes that are smaller than the stripe size such as database access the drives will be able to seek to independently. If the sectors accessed are spread evenly between the two drives then the apparent seek time of the

array will be half that of a single non raid drive (assuming identical disks in the array). The transfer speed of the array will be the transfer speed of all the disks added together, limited only by the speed of the RAID controller. Note that these performance scenarios are in the best case with optimal access patterns. RAID 0 is useful for setups such as large read-only NFS servers where mounting many disks is time-consuming or impossible and redundancy is irrelevant. Another use is where the number of disks is limited by the operating system. In Microsoft Windows, the number of drive letters for hard disk drives may be limited to 24, so RAID 0 is a popular way to use more disks. It is possible in Windows 2000 Professional and newer to mount partitions under directories, much like Unix, and hence eliminating the need for a partition to be assigned a drive letter. RAID 0 is also a popular choice for gaming systems where performance is desired, data integrity is not very important, but cost is a consideration to most users. However, since data is shared between drives without redundancy, hard drives cannot be swapped out as all disks are dependent upon each other. NOTE: Some sites have stated that for home PCs, the speed advantages are debatable.


Concatenation (JBOD)

Diagram of a JBOD setup with 3 unequally-sized disks Although a concatenation of disks (also called JBOD, or "Just a Bunch Of Disks") is not one of the numbered RAID levels, it is a popular method for combining multiple physical disk drives into a single virtual one. As the name implies, disks are merely concatenated together, end to beginning, so they appear to be a single large disk. Concatenation may be thought of as the reverse of partitioning. Whereas partitioning takes one physical drive and creates two or more logical drives, JBOD uses two or more physical drives to create one logical drive. In that it consists of an Array of Independent Disks (no redundancy), it can be thought of as a distant relation to RAID. JBOD is sometimes used to turn several odd-sized drives into one larger useful drive, which cannot be done with RAID 0. For example, JBOD could use a 3 GB, 15 GB, 5.5 GB, and 12 GB drive to combine into a logical drive at 35.5 GB, which is often more useful than the individual drives separately. In the diagram to the right, data is concatenated from the end of disk 0 (block A63) to the beginning of disk 1 (block A64); end of disk 1 (block A91) to the beginning of disk 2 (block A92). If RAID 0 were used, then disk 0 and disk 2 would be truncated to 28 blocks (disk 1 contains 28 blocks) for a total size of 84 blocks. JBOD is similar to the widely used Logical Volume Manager (LVM) and Logical Storage Manager (LSM) in UNIX and UNIX-based operating systems (OS). JBOD is useful for OSs which do not support LVM/LSM (like MS-Windows, although Windows Server 2003, Windows XP Pro, and Windows 2000 support software JBOD, known as spanning dynamic disks). The difference between JBOD and LVM/LSM is that the address remapping between the logical address of the concatenated device and the physical address of the disc is done by the RAID hardware instead of the OS kernel as it is LVM/LSM. One advantage JBOD has over RAID 0 is in the case of drive failure. Whereas in RAID 0, failure of a single drive will usually result in the loss of all data in the array, in a JBOD array only the data on the affected drive is lost, and the data on surviving drives will remain readable. However, JBOD does not carry the performance benefits which are associated with RAID 0. This does not address file system coherency as a loss of a large portion of the file system will likely render it inoperable, but raw data access to the surviving disks will yield readable data because the data has not be striped acrossed to the failed disk. Note: Some Raid cards (e.g. 3ware) use JBOD to refer to configuring drives without raid features including concatenation. Each drive shows up separately in the OS.

Note: Many Linux distributions refer to JBOD as "linear mode" or "append mode." The Mac OS 10.4 implementation - called a "Concatenated Disk Set" - does NOT leave the user with any usable data on the remaining drives if one drive fails in a "Concatenated Disk Set," although the disks do have the write performance documented in the illustration above. RAID 1 A RAID 1 creates an exact copy (or mirror) of a set of data on two or more disks. This is useful when read performance or reliability are more important than data capacity. Such an array can only be as big as the smallest member disk. A classic RAID 1 mirrored pair contains two disks (see diagram), which increases reliability exponentially over a single disk. Since each member contains a complete copy of the data, and can be addressed independently, ordinary wear-and-tear reliability is raised by the power of the number of self-contained copies. RAID 1 failure rate For example, consider a RAID 1 with two identical models of a disk drive with a weekly probability of failure of 1:500. Assuming defective drives are replaced weekly, the installation would carry a 1:250,000 probability of failure for a given week. That is, the likelihood that the RAID array is down due to mechanical failure during any given week is the product of the likelihoods of failure of both drives. In other words, the probability of failure is 1 in 500 and if the failures are statistically independent then the probability

of both drives failing is


This is purely theoretical however, the chance of a failure is much higher because drives are often manufactured at the same time and subjected to the same stresses. If a failure is because of an environmental problem, it's quite likely that the other drive will fail shortly after the first. RAID 1 performance Additionally, since all the data exists in two or more copies, each with its own hardware, the read performance can go up roughly as a linear multiple of the number of copies. That is, a RAID 1 array of two drives can be reading in two different places at the same time, though not all implementations of RAID 1 do this[3]. To maximize performance benefits of RAID 1, independent disk controllers are recommended, one for each disk. Some refer to this practice as splitting or duplexing. When reading, both disks can be accessed independently and requested sectors can be split evenly between the disks. For the usual mirror of two disks this would double the transfer rate. The apparent access time of the array would be half that of a single non raid drive. Unlike RAID 0 this would be for all access patterns as all the data is present on all the disks. Read performance can be further improved by adding drives to the mirror. Three disks would give you three times the throughput and an apparent seek time of a third. The only limit is how many disks can be connected to the controller and its maximum transfer speed. Many older IDE RAID 1 cards read from one disk in the pair, so their read performance is that of a single disk. Some older RAID 1 implementations would also read both disks simultaneously and compare the data to catch errors. The error detection and correction on modern disks makes this less useful in environments requiring normal commercial availability. When writing, the array performs like a single disk as all mirrors must be written with the data. Note that these performance scenarios are in the best case with optimal access patterns. RAID 1 has many administrative advantages. For instance, in some 365/24 environments, it is possible to "Split the Mirror": declare one disk as inactive, do a backup of that disk, and then "rebuild" the mirror. This requires that the application support recovery from the image of data on the disk at the point of the mirror split. This procedure is less critical in the presence of the "snapshot" feature of some filesystems, in which some space is reserved for changes, presenting a static point-in-time view of the filesystem. Alternatively, a set of disks can be kept in much the same way as traditional backup tapes are. RAID 2 A RAID 2 stripes data at the bit (rather than block) level, and uses a Hamming code for error correction. The disks are synchronized by the controller to spin in perfect tandem. This is the only original level of RAID that is not currently used. Extremely high data transfer rates are possible. The use of the Hamming(7,4) code also permits using 7 disks in RAID 2, with 4 being used for data storage and 3 being used for error correction. RAID 3

Diagram of a RAID 3 setup of 6-byte blocks and two parity bytes, shown are two blocks of data (orange and green) A RAID 3 uses byte-level striping with a dedicated parity disk. RAID 3 is very rare in practice. One of the side-effects of RAID 3 is that it generally cannot service multiple requests simultaneously. This comes about because any single block of data will, by

definition, be spread across all members of the set and will reside in the same location. So, any I/O operation requires activity on every disk. In our example above, a request for block "A" consisting of bytes A1-A6 would require all three data disks to seek to the beginning (A1) and reply with their contents. A simultaneous request for block B would have to wait.

[edit] RAID 4

Diagram of a RAID 4 setup with a dedicated parity disk 3 with each color representing the stripe of blocks in the respective parity block A RAID 4 uses block-level striping with a dedicated parity disk. RAID 4 looks similar to RAID 5 except that it does not use distributed parity, and similar to RAID 3 except that it stripes at the block, rather than the byte level. This allows each member of the set to act independently when only a single block is requested. If the disk controller allows it, a RAID 4 set can service multiple read requests simultaneously. In our example, a request for block "A1" would be serviced by disk 1. A simultaneous request for block B1 would have to wait, but a request for B2 could be serviced concurrently.


Diagram of a RAID 5 setup with distributed parity with each color representing the group of blocks in the respective parity block (a stripe) A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5 has achieved popularity due to its low cost of redundancy. Generally, RAID 5 is implemented with hardware support for parity calculations. A minimum of 3 disks is generally required for a complete RAID 5 configuration (A RAID 5 two disk set is possible, but many implementations do not allow for this. In some implementations a degraded disk set can be made (3 disk set of which 2 are online)) In the example above, a read request for block "A1" would be serviced by disk 1. A simultaneous read request for block B1 would have to wait, but a read request for B2 could be serviced concurrently.

RAID 5 parity handling Every time a block is written to a disk in a RAID 5, a parity block is generated within the same stripe. A block is often composed of many consecutive sectors on a disk. A series of blocks (a block from each of the disks in an array) is collectively called a "stripe". If another block, or some portion of a block, is written on that same stripe, the parity block (or some portion of the parity block) is recalculated and rewritten. For small writes, this requires reading the old data, writing the new parity, and writing the new data. The disk used for the parity block is staggered from one stripe to the next, hence the term "distributed parity blocks". RAID 5 writes are expensive in terms of disk operations and traffic between the disks and the controller. The parity blocks are not read on data reads, since this would be unnecessary overhead and would diminish performance. The parity blocks are read, however, when a read of a data sector results in a cyclic redundancy check (CRC) error. In this case, the sector in the same relative position within each of the remaining data blocks in the stripe and within the parity block in the stripe are used to reconstruct the errant sector. The CRC error is thus hidden from the main computer. Likewise, should a disk fail in the array, the parity blocks from the surviving disks are combined mathematically with the data blocks from the surviving disks to reconstruct the data on the failed drive "on the fly". This is sometimes called Interim Data Recovery Mode. The computer knows that a disk drive has failed, but this is only so that the operating system can notify the administrator that a drive needs replacement; applications running on the computer are unaware of the failure. Reading and writing to the drive array continues seamlessly, though with some performance degradation. The difference between RAID 4 and RAID 5 is that, in interim data recovery mode, RAID 5 might be slightly faster than RAID 4, because, when the CRC and parity are in the disk that failed, the calculation does not have to be performed, while with RAID 4, if one of the data disks fails, the calculations have to be performed with each access. In RAID 5, where there is a single parity block per stripe, the failure of a second drive results in total data loss. Whereas the Master Boot Record (MBR) table is written separately in all the Physical drives. RAID 5 disk failure rate The maximum number of drives in a RAID 5 redundancy group is theoretically unlimited, but it is common practice to limit the number of drives. The tradeoffs of larger redundancy groups are greater probability of a simultaneous double disk failure, the increased time to rebuild a redundancy group, and the greater probability of encountering an unrecoverable sector during RAID reconstruction. As the number of disks in a RAID 5 group increases, the MTBF (failure rate) can become lower than that of a single disk. This happens when the likelihood of a second disk failing out of (N-1) dependent disks, within the time it takes to detect, replace and recreate a first failed disk, becomes larger than the likelihood of a single disk failing. RAID 6 is an alternative that provides dual parity protection thus enabling larger numbers of disks per RAID group. Some RAID vendors will avoid placing disks from the same manufacturing lot in a redundancy group to minimize the odds of simultaneous early life and end of life failures as evidenced by the bathtub curve. RAID 5 performance RAID 5 implementations suffer from poor performance when faced with a workload which includes many writes which are smaller than the capacity of a single stripe; this is because parity must be updated on each write, requiring read-modify-write sequences for both the data block and the parity block. More complex implementations often include non-volatile write back cache to reduce the performance impact of incremental parity updates. The read performance of RAID 5 is almost as good as RAID 0 for the same number of discs. If you ignore the parity blocks the on disc layout looks exactly like that of RAID 0. The reason RAID 5 is slightly slower is that the disks must skip over the parity blocks. In the event of a system failure while there are active writes, the parity of a stripe may become inconsistent with the data. If this is not detected and repaired before a disk or block fails, data loss may ensue as incorrect parity will be used to reconstruct the missing block in that stripe. This potential vulnerability is sometimes known as the "write hole." Battery-backed cache and other techniques are commonly used to reduce the window of vulnerability of this occurring. RAID 5 usable size

The user capacity of a RAID 5 array is , where N is the total number of drives in the array and Si is the capacity of the ith drive and, thus, Smin is the capacity of the smallest drive in the array. RAID 6 Diagram of a RAID 6 setup which is just like RAID 5 but with two parity blocks instead of one

A RAID 6 extends RAID 5 by adding an additional parity block, thus it uses block-level striping with two parity blocks distributed across all member disks. It was not one of the original RAID levels. RAID 5 can be seen as a special case of a Reed-Solomon code[4]. RAID 5, being a degenerate case, requires only addition in the Galois field. Since we are operating on bits, the field used is a binary galois field galois fields, addition is computed by a simple XOR.

. In cyclic representations of binary

After understanding RAID 5 as a special case of a Reed-Solomon code, it is easy to see that it is possible to extend the approach to produce redundancy simply by producing another syndrome; typically a polynomial in (8 means we are operating on bytes). By adding additional syndromes it is possible to achieve any number of redundant disks, and recover from the failure of that many drives anywhere in the array, but RAID 6 refers to the specific case of two syndromes. Like RAID 5 the parity is distributed in stripes, with the parity blocks in a different place in each stripe. RAID 6 performance RAID 6 is inefficient when used with a small number of drives but as arrays become bigger and have more drives the loss in storage capacity becomes less important and the probability of two disks failing at once is bigger. RAID 6 provides protection against double disk failures and failures while a single disk is rebuilding. In the case where there is only one array it may make more sense than having a hot spare disk.

The user capacity of a RAID 6 array is , where N is the total number of drives in the array, Si is the capacity of the ith, and Smin is the capacity of the smallest drive in the array. RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations due to the overhead associated with the additional parity calculations. This penalty can be minimized by coalescing writes in fewer stripes, which can be achieved by a Write Anywhere File Layout. RAID 6 implementation According to SNIA (Storage Networking Industry Association), the definition of RAID 6 is: "Any form of RAID that can continue to execute read and write requests to all of a RAID array's virtual disks in the presence of any two concurrent disk failures. Several methods, including dual check data computations (parity and Reed Solomon), orthogonal dual parity check data and diagonal parity have been used to implement RAID Level 6." RAID 5E, RAID 5EE and RAID 6E RAID 5E, RAID 5EE and RAID 6E generally refer to variants of RAID 5 or RAID 6 with online (hot) spare drives, where the spare drives are an active part of the block rotation scheme. This allows the I/O to be spread across all drives, including the spare, thus reducing the I/O bandwidth per drive, allowing for higher performance. It does, however, mean that a spare drive cannot be shared among multiple arrays, which is occasionally desirable. In RAID 5E, RAID 5EE and RAID 6E, there is no dedicated "spare drive", just like there is no dedicated "parity drive" in RAID 5 or RAID 6. Instead, the spare blocks are distributed across all the drives, so that in a 10-disk RAID 5E with one spare, each and every disk is 80% data, 10% parity, and 10% spare. The spare blocks in RAID 5E and RAID 6E are at the end of the array, while in RAID 5EE the spare blocks are integrated into the array. RAID 5EE level can sustain a single drive failure. RAID 5EE requires at least four disks and can expand up to 16 disks.

Nested RAID levels From Wikipedia, the free encyclopedia Jump to: navigation, search Main article: RAID To gain performance and/or additional redundancy the Standard RAID levels can be combined to create hybrid or Nested RAID levels. Contents

1 Nesting

• • •

2 RAID 0+1

• • •

5 RAID 100 (RAID 10+0)

8 See also

3 RAID 10 4 Raid 0+3 and 3+0 o 4.1 RAID 0+3 o 4.2 RAID 30 6 RAID 50 (RAID 5+0) 7 RAID 60 (RAID 6+0)

Nesting When nesting RAID levels, a RAID type that provides redundancy is typically combined with RAID 0 to boost performance. With these configurations it is preferable to have RAID 0 on top and the redundant array at the bottom, because fewer disks then need to be regenerated when a disk fails. (Thus, RAID 10 is preferable to RAID 0+1 but the administrative advantages of "splitting the mirror" of RAID 1 would be lost). RAID 0+1

Block diagram of a RAID 0+1 setup. A RAID 0+1 (also called RAID 01, not to be confused with RAID 1), is a RAID used for both replicating and sharing data among disks. The difference between RAID 0+1 and RAID 1+0 is the location of each RAID system — RAID 0+1 is a mirror of stripes. Consider an example of RAID 0+1: six 120 GB drives need to be set up on a RAID 0+1. Below is an example where two 360 GB level 0 arrays are mirrored, creating 360 GB of total storage space: RAID 1 .--------------------------. | | RAID 0 RAID 0 .-----------------. .-----------------. | | | | | | 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB A1 A2 A3 A1 A2 A3 A4 A5 A6 A4 A5 A6 A7 A8 A9 A7 A8 A9 A10 A11 A12 A10 A11 A12 Note: A1, A2, et cetera each represent one data block; each column represents one disk. The maximum storage space here is 360 GB, spread across two arrays. The advantage is that when a hard drive fails in one of the level 0 arrays, the missing data can be transferred from the other array. However, adding an extra hard drive to one stripe requires you to add an additional hard drive to the other stripes to balance out storage among the arrays. It is not as robust as RAID 10 and cannot tolerate two simultaneous disk failures, unless the second failed disk is from the same stripe as the first. That is, once a single disk fails, each of the mechanisms in the other stripe is single point of failure. Also, once the single failed mechanism is replaced, in order to rebuild its data all the disks in the array must participate in the rebuild. The exception to this is if all the disks are hooked up to the same raid controller in which case the controller can do the same error recovery as RAID 10 as it can still access the functional disks in each RAID 0 set. If you compare the diagrams between RAID 0+1

and RAID 10 and ignore the lines above the disks you will see that all that's different is that the disks are swapped around. If the controller has a direct link to each disk it can do the same. In this one case there is no difference between RAID 0+1 and RAID 10. With increasingly larger capacity disk drives (driven by serial ATA drives), the risk of drive failure is increasing. Additionally, bit error correction technologies have not kept up with rapidly rising drive capacities, resulting in higher risks of encountering media errors. In the case where a failed drive is not replaced in a RAID 0+1 configuration, a single uncorrectable media error occurring on the mirrored hard drive would result in data loss. Given these increasing risks with RAID 0+1, many business and mission critical enterprise environments are beginning to evaluate more fault tolerant RAID setups that add underlying disk parity. Among the most promising are hybrid approaches such as RAID 0+1+5 (mirroring above single parity) or RAID 0+1+6 (mirroring above dual parity). RAID 10

Diagram of a RAID 10 setup. A RAID 10, sometimes called RAID 1+0, or RAID 1&0, is similar to a RAID 0+1 with exception that the RAID levels used are reversed — RAID 10 is a stripe of mirrors. Below is an example where three collections of 120 GB level 1 arrays are striped together to make 360 GB of total storage space: RAID 0 .-----------------------------------. | | | RAID 1 RAID 1 RAID 1 .--------. .--------. .--------. | | | | | | 120 GB 120 GB 120 GB 120 GB 120 GB 120 GB A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6 A7 A7 A8 A8 A9 A9 A10 A10 A11 A11 A12 A12 Note: A1, A2, et cetera each represent one data block; each column represents one disk. All but one drive from each RAID 1 set could fail without damaging the data. However, if the failed drive is not replaced, the single working hard drive in the set then becomes a single point of failure for the entire array. If that single hard drive then fails, all data stored in the entire array is lost. As is the case with RAID 0+1, if a failed drive is not replaced in a RAID 10 configuration then a single uncorrectable media error occurring on the mirrored hard drive would result in data loss. Some RAID 10 vendors address this problem by supporting a "hot spare" drive, which automatically replaces and rebuilds a failed drive in the array. Given these increasing risks with RAID 10, many business and mission critical enterprise environments are beginning to evaluate more fault tolerant RAID setups that add underlying disk parity. Among the most promising are hybrid approaches such as RAID 0+1+5 (mirroring above single parity) or RAID 0+1+6 (mirroring above dual parity). RAID 10 is often the primary choice for high-load databases, because the lack of parity to calculate gives it faster write speeds. RAID 10 Capacity: (Size of Smallest Drive) * (Number of Drives) / 2 The Linux kernel RAID10 implementation (from version 2.6.9 and onwards) is not nested. The mirroring and striping is done in one process. Only certain layouts are standard RAID 10 with the rest being proprietary. See the Linux MD RAID 10 section in the Nonstandard RAID article for details.

Raid 0+3 and 3+0 RAID 0+3

Diagram of a 0+3 array RAID level 0+3 or RAID level 03 is a dedicated parity array across striped disks. Each block of data at the RAID 3 level is broken up amongst RAID 0 arrays where the smaller pieces are striped across disks.

RAID 30 RAID level 30 is also known as striping of dedicated parity arrays. It is a combination of RAID level 3 and RAID level 0. RAID 30 provides high data transfer rates, combined with high data reliability. RAID 30 is best implemented on two RAID 3 disk arrays with data striped across both disk arrays. RAID 30 breaks up data into smaller blocks, and then stripes the blocks of data to each RAID 3 raid set. RAID 3 breaks up data into smaller blocks, calculates parity by performing an Exclusive OR on the blocks, and then writes the blocks to all but one drive in the array. The parity bit created using the Exclusive OR is then written to the last drive in each RAID 3 array. The size of each block is determined by the stripe size parameter, which is set when the RAID is created. Advantages One drive from each of the underlying RAID 3 sets can fail. Until the failed drives are replaced the other drives in the sets that suffered such a failure are a single point of failure for the entire RAID 30 array. In other words, if one of those drives fails, all data stored in the entire array is lost. The time spent in recovery (detecting and responding to a drive failure, and the rebuild process to the newly inserted drive) represents a period of vulnerability to the RAID set. Offers highest level of redundancy and performance Disadvantages Very costly to implement /------/------/------/------> RAID CONTROLLER
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