Redundant Array of Inexpensive Disks

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RedundantArray of InexpensiveDisks

• Stephen Clarke
• CS 215
• Dr. Joel

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History

• An acronym first defined by David A. Patterson,
  Garth A. Gibson and Randy Katz at the University
  of California, Berkeley in 1987 to describe a
  Redundant Array of Inexpensive Disks,
• Technology that allowed computer users to achieve
  high levels of storage reliability from low-cost
  and less reliable PC-class disk-drive components,
  via the technique of arranging the devices into
  arrays for redundancy.

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Also Known as

• Redundant Array of Independent Disks
• Inexpensive changed to Independent to allow for
  more profit.

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What is RAID?

• Technology that allows users to achieve high
  levels of storage reliability from low-cost and
  less reliable PC-class disk-drive components
• Done via the technique of arranging the devices
  into arrays for redundancy.

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Purposes

• Redundancy writing the same data to multiple
  drives (known as mirroring), or writing extra
  data (known as parity data) across the array,
  calculated such that the failure of one (or
  possibly more, depending on the type of RAID)
  disks in the array will not result in loss of
  data.
• So Its a form of preventing data loss by
  creating back ups of the data

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How RAID works

• RAID combines two or more physical hard disks
  into a single logical unit by using either
  special hardware or software. Hardware solutions
  often are designed to present themselves to the
  attached system as a single hard drive, so that
  the operating system would be unaware of the
  technical workings. For example, you might
  configure a 1TB RAID 5 array using three 500GB
  hard drives in hardware RAID, the operating
  system would simply be presented with a "single"
  1TB disk. Software solutions are typically
  implemented in the operating system and would
  present the RAID drive as a single drive to
  applications running upon the operating system.

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Key Design Goals

• Increased data reliability a property of some
  disk arrays which provides fault tolerance, so
  that all or part of the data stored in the array
  can be recovered in the case of disk failure.
• Increased input/output performance.

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Raid Array

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

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Single and Dual Parity

• Large-capacity drives lengthen the time needed to
  recover from the failure of a single drive.
• Single parity RAID levels are vulnerable to data
  loss until the failed drive is rebuilt the
  larger the drive, the longer the rebuild will
  take.
• Dual parity gives time to rebuild the array
  without the data being at risk if a (single)
  additional drive fails before the rebuild is
  complete.

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Common RAID Levels

• RAID 0 - Striped set without parity
• RAID 1 - Mirrored settings/disks
• RAID 5 - Striped disks with parity
• RAID 6 - Striped disks with dual parity
• RAID 10 - both striping and mirroring

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Striping

• The splitting of data across more than one disk.
• Segments can be assigned to multiple physical
  devices (usually disk drives in the case of RAID
  storage) in a round-robin fashion and thus
  written concurrently.
• This technique is useful if the processor is
  capable of reading or writing data faster than a
  single disk can supply or accept it. While data
  is being transferred from the first disk, the
  second disk can locate the next segment. Striping
  can be either of type coarse or fine.

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RAID 0

• "Striped set without parity" or "Striping"
• Provides improved performance and additional
  storage but no redundancy or fault tolerance.
• Any disk failure destroys the array, which
  becomes more likely with more disks in the array.
  A single disk failure destroys the entire array
  because when data is written to a RAID 0 drive,
  the data is broken into fragments.
• 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.

The number of fragments is dictated by the number
of disks in the array. 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, increasing bandwidth.
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RAID 1

• Duplicates data across every disk in the array,
  providing full redundancy.
• Two (or more) disks each store exactly the same
  data, at the same time, and at all times.
• Data is not lost as long as one disk survives.
• Total capacity of the array equals the capacity
  of the smallest disk in the array.
• At any given instant, the contents of each disk
  in the array are identical to that of every other
  disk in the array.

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RAID 5

• Distributed parity requires all drives but one to
  be present to operate
• Drive failure requires replacement, but the array
  is not destroyed by a single drive failure. Upon
  drive failure, any subsequent reads can be
  calculated from the distributed parity such that
  the drive failure is masked from the end user.
• The array will have data loss in the event of a
  second drive failure and is vulnerable until the
  data that was on the failed drive is rebuilt onto
  a replacement drive.
• A single drive failure in the set will result in
  reduced performance of the entire set until the
  failed drive has been replaced and rebuilt.

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RAID 6

• Less common
• Provides fault tolerance from two drive failures
• Array continues to operate with up to two failed
  drives.
• More practical for larger RAID groups

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Raid 10

• RAID 10 (or 10) uses both striping and
  mirroring. "01" or "01" is sometimes
  distinguished from "10" or "10" a striped set
  of mirrored subsets and a mirrored set of striped
  subsets are both valid, but distinct,
  configurations.

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Benefits of using RAID

• Higher Data Security
• Fault Tolerance
• Improved Availability
• Increased, Integrated Capacity
• Improved Performance

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Higher Data Security

• Through the use of redundancy, most RAID levels
  provide protection for the data stored on the
  array.
• This means that the data on the array can
  withstand even the complete failure of one hard
  disk (or sometimes more) without any data loss,
  and without requiring any data to be restored
  from backup.
• This security feature is a key benefit of RAID
  and probably the aspect that drives the creation
  of more RAID arrays than any other.
• All RAID levels provide some degree of data
  protection, depending on the exact
  implementation, except RAID level 0.

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Fault Tolerance

• RAID implementations that include redundancy
  provide a much more reliable overall storage
  subsystem than can be achieved by a single disk.
  This means there is a lower chance of the storage
  subsystem as a whole failing due to hardware
  failures. (At the same time though, the added
  hardware used in RAID means the chances of having
  a hardware problem of some sort with an
  individual component, even if it doesn't take
  down the storage subsystem, is increased see
  this full discussion of RAID reliability for
  more.)

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Improved Availability

• Availability refers to access to data. Good RAID
  systems improve availability both by providing
  fault tolerance and by providing special features
  that allow for recovery from hardware faults
  without disruption. See the discussion of RAID
  reliability and also this discussion of advanced
  RAID features.

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Integrated Capacity

• By turning a number of smaller drives into a
  larger array, you add their capacity together
  (though a percentage of total capacity is lost to
  overhead or redundancy in most implementations).
  This facilitates applications that require large
  amounts of contiguous disk space, and also makes
  disk space management simpler.

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Improved Performance

• RAID systems improve performance by allowing the
  controller to exploit the capabilities of
  multiple hard disks to get around
  performance-limiting mechanical issues that
  plague individual hard disks. Different RAID
  implementations improve performance in different
  ways and to different degrees, but all improve it
  in some way. See this full discussion of RAID
  performance issues for more.

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Disadvantages

• Less usable storage capacity For instance, a
  2-disk RAID 1 array loses half of the total
  capacity that would have otherwise been available
  using both disks independently, and a RAID 5
  array with several disks loses the capacity of
  one disk. Other types of RAID arrays are arranged
  so that they are faster to write to and read from
  than a single disk

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Disadvantages (Continued)

• When the bad disk is replaced by a new one the
  array is rebuilt while the system continues to
  operate normally. Some systems have to be powered
  down when removing or adding a drive others
  support hot swapping, allowing drives to be
  replaced without powering down. RAID with
  hot-swapping is often used in high availability
  systems, where it is important that the system
  remains running as much of the time as possible.
• RAID is NOT a good alternative to backing up
  data. Data may become damaged or destroyed
  without harm to the drive(s) on which they are
  stored. For example, part of the data may be
  overwritten by a system malfunction.
• A file may be damaged or deleted by user error or
  malice and not noticed for days or weeks
• The entire array is at risk of physical damage.

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More History

• Now used as an umbrella term for computer data
  storage schemes that can divide and replicate
  data among multiple hard disk drives.

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History

• Norman Ken Ouchi at IBM was awarded a 1978 U.S.
  patent 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.

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History (Continued)

• 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.
• 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.

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Further Reading

• Wikipedias article on RAID contains a detailed
  entry and many useful sources and citations
• http//en.wikipedia.org/wiki/RAID
• http//en.wikipedia.org/wiki/Standard_RAID_levels