R.A.I.D.
Redundant Array of Independent
(or Inexpensive) Disks
Copyright © 2005 by James Hug
Redundant Array of Independent
(or Inexpensive) Disks
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In 1987, Patterson, Gibson and Katz at the University of California Berkeley,
published a paper entitled "A Case for Redundant Arrays of Inexpensive Disks
(RAID)" . This paper described various types of disk arrays, referred to by the
acronym RAID. The basic idea of RAID was to combine multiple small,
inexpensive disk drives into an array of disk drives which yields performance
exceeding that of a Single Large Expensive Drive (SLED). Additionally, this
array of drives appears to the computer as a single logical storage unit or
drive.
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Five types of array architectures, RAID-1 through RAID-5, were defined by the
Berkeley paper, each providing disk fault-tolerance and each offering different
trade-offs in features and performance. In addition to these five redundant
array architectures, it has become popular to refer to a non-redundant array of
disk drives as a RAID-0 array.
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Redundant Array of Independent
(or Inexpensive) Disks
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Striping
Spreading out blocks of each file across multiple disk drives.
► Advantages:
 I/O performance is greatly improved by spreading the I/O load
across many channels and drives.
► Disadvantages:
 Not a "True" RAID because it is NOT fault-tolerant.
 The failure of just one drive will result in all data in an array being
lost.
 Should never be used in mission critical environments.
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Redundant Array of Independent
(or Inexpensive) Disks
► Mirroring
Duplicating blocks of each file across several disk drives.
► Advantages:
 100% redundancy of data means no data is lost and no rebuild is
necessary in case of a disk failure, just a copy to the replacement
disk.
► Disadvantages:
 Twice the Read transaction rate of single disks, same Write
transaction rate as single disks.
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
0 -- Striped Disk Array without Fault
Tolerance:
 Provides data striping (spreading out blocks of each file across
multiple disk drives) but no redundancy. This improves performance
but does not deliver fault tolerance.
 If one drive fails then all data in the array is lost.
 Requires a minimum of two (2) hard disks.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
1 -- Mirroring and Duplexing:
 Data is written to two duplicate disks simultaneously. This way if
one of the disk drives fails, the system can instantly switch to the
other disk without any loss of data or service.
 Disk mirroring is used commonly in mission critical systems where
it's critical that the data be accessible at all times.
 Requires a minimum of two (2) hard disks.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
0+1 -- A Mirror of Stripes:
 Not one of the original RAID levels, two RAID 0 stripes are created,
and a RAID 1 mirror is created over them.
 Used for both replicating and sharing data among disks.
 RAID 0+1 has the same fault tolerance as RAID level 5.
 RAID 0+1 has the same overhead for fault-tolerance as mirroring
alone.
 Requires a minimum of four (4) hard disks
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
2 -- Error-Correcting Coding (ECC):
 Not a typical implementation and rarely used, Level 2 stripes data
at the bit level rather than the block level.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
3 -- Bit-Interleaved Parity:
 Provides byte-level striping with a dedicated parity disk. Level 3,
which cannot service simultaneous multiple requests, also is rarely
used.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
4 -- Dedicated Parity Drive:
 A commonly used implementation of RAID, Level 4 provides blocklevel striping (like Level 0) with a parity disk. If a data disk fails, the
parity data is used to create a replacement disk. A disadvantage to
Level 4 is that the parity disk can create write bottlenecks.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
5 -- Block Interleaved Distributed Parity:
 Provides data striping at the byte level and also stripe error
correction information. This results in excellent performance and
good fault tolerance.
 Level 5 is one of the most popular implementations of RAID used in
corporate file server installations.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
6 -- Independent Data Disks with Double
Parity:
 Provides block-level striping with parity data distributed across all
disks.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
10 -- A Stripe of Mirrors:
 Not one of the original RAID levels, multiple RAID 1 mirrors are
created, and a RAID 0 stripe is created over these.
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Redundant Array of Independent
(or Inexpensive) Disks
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Redundant Array of Independent
(or Inexpensive) Disks
► Level
7: A trademark of Storage Computer
► RAID
S: EMC Corporation's proprietary striped
Corporation that adds caching to Levels 3 or 4.
parity RAID system used in its Symmetrix storage
systems.
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Redundant Array of Independent
(or Inexpensive) Disks
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Most RAID systems are managed at the hardware level and
are invisible to the operating system.
A RAID. system requires the appropriate hard disk
controller (card).
Some motherboards include Level 1, Level 0, and/or Level
1+0.
There are RAID. systems for SATA, IDE/ATA, and SCSI hard
drives.
Motherboards that have RAID capabilities also have the
standard disk controller, which can be used conventionally.
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Redundant Array of Independent
(or Inexpensive) Disks
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Hard drives in a RAID. system should be matched for size
and speed.
In un-matched systems the smaller/slower hard drive will
be the controlling factor.
Naturally with more hard drives (minimum of two) the
more expensive the computer.
In a striped system the storage capacity is twice the
smallest hard drive.
In a mirrored system the storage capacity is the same as
the smallest hard drive.
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