Raid

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RAID Technology Definition of RAID RAID, or Redundant Array of Inexpensive Disks (or later also referred to as Redundant Array of Independent Disks) is an acronym first used in a 1988 paper by Berkeley researchers Patterson, Gibson and Katz. RAID is a technology developed to improve data protection and performance while storing large amounts of data, without necessarily requiring improvements in disk drive technology. RAID Levels As the definition and awareness of the RAID technology has grown,
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  RAID Technology Definition of RAID  RAID, or Redundant Array of Inexpensive Disks (or later also referred to asRedundant Array of Independent Disks) is an acronym first used in a 1988 paper byBerkeley researchers Patterson, Gibson and Katz. RAID is a technology developed toimprove data protection and performance while storing large amounts of data,without necessarily requiring improvements in disk drive technology. RAID Levels  As the definition and awareness of the RAID technology has grown, several RAIDconfigurations for storing data have been devised and standardized upon. TheseRAID levels are now commonly discussed in the industry. The simplest RAIDconfigurations either stripe data across two drives to increase data transfer speed,but offer no data protection; or mirror redundant data onto a second drive, withoutincreasing performance. More advanced configurations involve three or more drives,and simultaneously provide fault tolerance, increased performance, and the ability to recreate information onto a spare drive should a drive failure occur. These moreadvanced RAID configurations are preferred in server environments where maximumdata availability and performance is critical.The applications, advantages, and disadvantages of the different RAIDconfigurations, or levels, are described below. The numbers assigned to each level of RAID do not indicate superiority or effectiveness; they are only used to differentiatebetween them. RAID 0 - Disk Striping  With RAID 0, or a configuration known as data striping , data is written insequential sections across more than two drives. RAID 0 is easy to implement, and itcan dramatically improve performance. Several drives can be accessed at once,minimizing the overall seek time of larger files. This configuration has no dataredundancy and therefore no protection against data loss, however, so it should notbe used for business-critical applications.  RAID 1 - Mirroring  Also known as drive mirroring , RAID 1 simultaneously copies data to a seconddrive. The mirroring method offers data protection and good performance in the casewhere a mirrored drive fails. RAID 1 is the simplest RAID configuration, requiringonly a minimum of two drives with equal capacity, and also that the drives be addedin pairs. The main disadvantage of RAID 1 is that it uses 100% drive overhead (thehighest of all RAID levels), which can be considered an inefficient use of drivecapacity. RAID 2- Redundancy Using Hamming Code  A RAID 2 array stripes data to a group of drives using a byte stripe. A hamming codeError Checking and Correction (ECC) symbol for each data stripe is stored on adedicated drive. This code provides detection and correction of data errors, allowingdata to be recovered without completely duplicating the data. Since most drives nowembed ECC information within each sector as standard, however, RAID 2 doesn'toffer any advantages over RAID 3. RAID 3 - Striping Plus Parity  RAID 3 stripes data across multiple drives, with an additional drive dedicated toparity for error correction/recovery. This configuration offers very high data transferrates and only requires a small percentage of ECC (parity) to data drives. However,RAID 3 requires a complicated controller design and the configuration may bedifficult to rebuild after a drive failure.  RAID 4 - Independent Striping Plus Parity  RAID 4 is identical to RAID 3 except that large strips are used, so that records canbe read from any individual drive in the array apart from the parity drive, allowingread operations to be overlapped. Since RAID 4 offers no significant advantages overRAID 5, the RAID 4 configuration is now rarely implemented. RAID 5 - Independent Striping Plus Distributed Parity  With RAID 5, each block of data is written on a data drive and parity information isthen striped across all drives. RAID 5 is the most popular of the RAID levels becauseit delivers data protection and good performance with a small overhead for parity.RAID 5 offers the most efficient use of drive capacity of all the redundant RAIDlevels. This configuration requires at least three drives of equal size, which can beadded one at a time. RAID 6 -RAID 5 With Double Parity (or P+Q Redundancy ) (Not recognized by the RAID Advisory Board (RAB).)  RAID 6 is an extension of RAID 5 that uses a second independent distributed parityscheme. Data is striped on a block level across a set of drives, and then a second setof parity is calculated and written across all of the drives. This configuration providesextremely high fault tolerance and can sustain several simultaneous drive failures,but it requires an n+2 number of drives and a very complicated controller design.  RAID 10 - Combination of RAID 1 and RAID 0 (Not recognized by srcinal Berkeley papers or by the RAB.)  RAID 10 combines RAID 0 and RAID 1 by striping data across multiple drives withoutparity, and it mirrors the entire array to a second set of drives. This process deliversfast data access (like RAID 0) and single drive fault tolerance (like RAID 1), but cutsthe usable drive space in half. RAID 10 requires a minimum of four equally sizeddrives, is the most expensive RAID solution and offers limited scalability. RAID 53 - Combination of RAID 0 and RAID 3 (Not recognized by srcinal Berkeley papers or by the RAB.)  The RAID 53 configuration should really be called RAID 03 . This configuration is astriped array whose segments are essentially RAID 3 arrays. It has the same faulttolerance and high data transfer rates of RAID 3, with the high I/O rates associatedwith RAID 0 (striping), plus some added performance. This configuration is veryexpensive and requires at least 5 drives to implement. RAID Applications, Tradeoffs and Limitations  Typically, RAID is used in systems where data accessibility is critical and faulttolerance is required, such as in large file servers. However, RAID is now also morefrequently seen used in desktop systems for CAD, multimedia editing and playbackwhere higher transfer rates are needed.In general, for a given price point, the performance improvement of a particular typeof RAID array trades off with the amount of the redundancy and data security of the array. Similarly, capacity of the array trades off with the price and faulttolerance. Inexpensive RAID solutions are limited in their ability to protect your dataor improve performance, whereas high-end RAID implementations providing veryhigh performance and very high data reliability are quite expensive.Although RAID can greatly improve the reliability and performance of a storagesystem, it is dangerous to assume that a RAID system with redundancy providesabsolute data protection. Since there are sources of failure that are still applicable toRAID systems, such as viruses, environmental disturbances and/or cases wheremore than one drive fails at the same time, regular system maintenance and backupremain critical practices.
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