RAID Outline
Short for Redundant Array of Independent (or Inexpensive) Disks,
a category of disk
drives that employ two or more drives in combination for fault tolerance
and performance. RAID disk
drives are used frequently on servers but aren’t
generally necessary for personal
computers. RAID allows you to store the same data redundantly (in multiple
paces) in a balanced ay to improve overall performance.
There
are number of different RAID levels:
·
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.
·
Level 1 — Mirroring and Duplexing:
Provides disk
mirroring. Level 1 provides twice the read transaction rate of single disks
and the same write transaction rate as single disks.
·
Level 2 — Error-Correcting Coding:
Not a typical implementation and rarely used, Level 2 stripes data at the bit
level rather than the block level.
·
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.
·
Level 4 — Dedicated Parity Drive:
A commonly used implementation of RAID, Level 4 provides block-level 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.
·
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.
·
Level 6 — Independent Data Disks with
Double Parity: Provides block-level striping with
parity data distributed across all 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.
·
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.
·
RAID S: (also
called Parity RAID) EMC Corporation’s proprietary striped parity RAID system used in its Symmetric
storage systems.
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