A r r a y B a s i c s
DataDirect Networks EF2800 FC RAID Storage System User Guide (V 1.0)
187
B.1.2
RAID 1, RAID 10 (Mirrored Disks)
In RAID 1 and RAID 10 arrays (commonly called
mirrored arrays
) disks
are paired, with both disks in a pair containing the same data. When
data is written to a mirrored array, it is written twice—once to each disk
in the pair. A RAID 1 array has only one set of paired disks. A RAID 10
array has multiple pairs, across which data is striped.
The read performance of RAID 1 arrays can be much better than that of
a single disk, while the write performance is slightly lower. In RAID 10
arrays, both read performance and write performance are better than
those of a single disk.
A mirrored array is also highly reliable, because both disks
in a pair
must fail for the array to fail. In an array with five pairs of mirrored
disks, for example, the array can maintain its integrity even if five disks
fail; as long as each pair is left with one good disk. The main
disadvantage of a mirrored array is its cost. Because all disks must have
a twin, you must use twice the number of disks that actually contribute
to the array capacity. In an eight-disk array, for example, you have only
four disks of usable capacity.
B.1.3
RAID 3
RAID 3 arrays contain redundant information in the form of parity data,
which is calculated block-by-block for all user data. The user data is
distributed across all but one of the disks in the array. The parity data
is written exclusively to the
parity disk
(also known as the
check disk
). In
the event of a disk failure, the data can be reconstructed from
corresponding data stripes on the remaining disks in the array.
RAID 3 provides excellent I/O performance for applications that
require high data transfer rates such as image processing, video
processing, scientific data collection, batch data processing, or
sequential reads and writes.
RAID 3 is not well suited for transaction processing or other
applications that require simultaneous reads and writes.
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