Raid

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Raid

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Raid

• RAID or Redundant Array of Inexpensive Disks is a
  technology that employs the simultaneous use of
  two or more hard disk drives to achieve greater
  levels of performance reliability, and/or larger
  data volume sizes.

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RAID

• RAID is now used as an umbrella term for computer
  data storage schemes that can divide and
  replicate data among multiple hard disk drives.
  RAIDs various designs all involve two key design
  goals, to increase data reliability and increase
  input/output performance.

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RAID

• When several physical disks are set up to use
  RAID technology, they are said to be in a RAID
  array. This array distributes data across several
  disks, but the array is seen by the computer user
  and operating system as one single disk.

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Types of raid

• Physical and logical arrays
• Mirroring
• Duplexing
• Striping
• Parity

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Arrays

• The fundamental structure of RAID is the array.
• An array is a collection of drives that is
  configured, formatted and manage in a particular
  way.
• The number of drives in the array, and the way
  that data is split between them, is what
  determines the Raid level, the capacity of the
  array, and its overall performance and data
  protection characteristics.

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Arrays

• Physical Drives The actual hard disks that
  comprise the array are the building blocks of all
  data storage under RAID
• Physical Arrays One or more physical drives are
  collected together to form a physical array.
• Most simple RAID setups use just one physical
  array. However, some complex ones can have two or
  more physical arrays.
• Logical arrays Logical arrays are formed by
  splitting or combining physical arrays.
  Typically, one logical array corresponds to one
  physical array. But, it is possible to set up a
  logical array that includes multiple physical
  arrays. It is also possible to set up two
  entirely different logical arrays from a single
  physical.
• Logical drives One or more logical drives are
  formed from one logical array. These appear to
  the operating system as if they were regular
  disk volumes, and are treated accordingly, with
  the RAID controller managing the array(s) that
  underlie them.

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Mirroring

• Mirroring is one of the two data redundancy
  techniques used in RAID
• In a RAID system using mirroring, all date in the
  system is written simultaneously to two hard
  disks instead of one thus the mirror concept.
  The principle behind mirroring is that this 100
  data redundancy provides full protection against
  the failure of either of he disks containing the
  duplicated data. Mirroring setups always require
  an even number of drives.
• The chief advantage of mirroring is that it
  provides not only complete redundancy of data,
  but also reasonably fast recovery from a disk
  failure. Since all the data is on the second
  drive, it is ready to use if the first one fails.
  Mirroring also improves some forms of read
  performance.
• The chief disadvantage of RAID 1 is expense that
  data duplication means half the space in the RAID
  is "wasted" so you must buy twice the capacity
  that you want to end up with in the array.
  Performance is also not as good as some RAID
  levels.

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Mirroring
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Duplexing

• Duplexing is an extension of mirroring that is
  based on the same principle as that technique.
  Like in mirroring, all data is duplicated onto
  two distinct physical hard drives, however in
  that it also duplicates the hardware that
  controls the two hard drives.
• If you were doing mirroring on two hard disks,
  they would both be connected to a single host
  adapter or RAID controller. If you were doing
  duplexing, one of the drives would be connected
  to one adapter and the other to a second adapter.
• Duplexing provides the same protection against
  drive failure that mirroring does, but also
  protects against the failure of either of the
  controllers.
• Duplexing costs more than mirroring because you
  are duplicating more hardware.
• Since hardware RAID is typically set up under the
  assumption that the RAID controller will handle
  all the drives in the array, duplexing is not
  supported as an option in most PC hardware RAID
  solutions. Duplexing is more often found in
  software RAID solutions managed by the operating
  system, where the operating system is running the
  RAID implementation at a high level and can
  easily split the data between the host adapters.

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Duplexing
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Striping

• For example, if we need to read a large file,
  instead of pulling it all from a single hard
  disk, it is much faster to chop it up into
  pieces, store some of the pieces on each of the
  drives in an array, and then use all the disks to
  read and back the file when needed.
• This technique is called striping, where you chop
  up pieces on the various drives with a different
  color used for each file.

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Striping

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Parity

• The Parity use in Raid is very similar to the
  Parity of RAM.
• In a parity N pieces of data are taken, and from
  them, the an extra piece of data is computed.
  Then take the N1 pieces of data and store them
  on N1 drives. If you lose any one of the N1
  pieces for data, you can recreate it from the N
  that remains, regardless of which piece is lost.
• Parity protection is used with striping, and the
  N pieces of data are typically the blocks or
  bytes distributed across the drives in the array.
  The parity information can either be stored on a
  separate, dedicated drive, or be mixed with the
  data across all the drives in the array.

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Raid levels

• There were five levels originally conceived, but
  many more variations have evolved, notably
  several nested levels and many non-standard
  levels.

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Raid Levels

• Here are some of the basic RAID levels
• RAID 0 or striped set without parity striping
• RAID 1 Mirrored set without parity mirroring
• Raid 2 Hamming code parity
• Raid 3 striped set with dedicated parity
• RAID 4 Block level parity
• RAID 5 striped set with distributive parity
• Raid 6 Stripped set with dual distributed parity

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History

• Norman Ken Ouchi at IBM was awarded a 1978 U.S
  patent 4092732 title System fo recovering data
  stored in failed memory unit. The claims for
  this patent describe what later be termed RAID 5
  with full stripe writes. This patent also
  mentions that disk mirroring or duplexing and
  protection with dedicated parity were prior art
  at this time.

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History

• 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 to or more
  drives to appear as a single device to the host
  system and published a paper. A Case of
  Redundant Arrays of Inexpensive Disks in June
  1988 at the SIGMOD conference.

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History

• Over the years different implementations of the
  RAID concepts have appeared. Most differ
  substantioally from the original idealized RAID
  Levels, But the numbered names remain.

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History

• IBM shipped RAID 1 in 1990, and it was not until
  then RAID had its breakthrough.
• In 1991 IBM launched RAID 3 and 1992 they
  released RAID 5.

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Manufacturers
Quantel AMCC
3pardata ANACAPA
Accordance AnecTEK
ACNC Apple
Adaptec Archion
Addonics Technologies Arco Computer Products
Adtron Arena MaxTronic
Advanced Media Services ARIO Data Networks
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Manufacturers
Aristos Logic Bridge Technology Cepoint Networks Condre Storage Data domain Dell Computer
Atrato Broadcom Chaparral Network Storage Coraid Dataman Benlux Digi-Data
ATTO technology Caen Engineering Ciprico Corporate systems Center Data Protection Solutions Dot Hill
Axstor Cambex Concurrent Computer Data Diredt Networks Data Storage Depot DsG Storage
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DTS
Dynamic Network Factory
EMC
Enhance Technology
European Storage Concept
ExaDrive N




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Sources

• http//en.wikipedia.org/wiki/Redundant_array_of_in
  dependent_disks
• http//www.howtofriends.com/raid/
• http//www.pcguide.com/ref/hdd/perf/raid/concepts/
  genArrays-c.html
• http//www.pcguide.com/ref/hdd/perf/raid/concepts/
  genMirroring-c.html

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Sources

• http//www.pcguide.com/ref/hdd/perf/raid/concepts/
  genDuplexing-c.html
• http//www.pcguide.com/ref/hdd/perf/raid/concepts/
  genStriping-c.html
• http//www.pcguide.com/ref/hdd/perf/raid/concepts/
  genParity-c.html