Raid 1, Raid 1/0 (Mirrored Disks); Raid 5 - Adaptec AHA-3980 User Manual

AHA-3980/3980W/3985/3985W User's Guide
RAID 0 arrays do not store redundant data, so they are not true
RAID applications. If one disk fails, the entire array fails and all array
data is lost. The fault-tolerance of a RAID 0 array, therefore, is less
than that of any single disk in the array. The term RAID 0 is widely
used for these arrays, however, because they are conceptually similar
to true RAID applications.

RAID 1, RAID 1/0 (Mirrored Disks)

In RAID 1 and RAID 1/0 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 1/0 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 worse. In
RAID 1/0 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 if even 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.

RAID 5

RAID 5 arrays contain redundant information in the form of parity
data, which is calculated block by block for all user data. The parity
information is distributed across the disks in the array, as in RAID 0
arrays, and occupies the equivalent capacity of about one disk. Data
is interspersed with the parity information. If one disk in the array
fails, the data on the failed disk can be reconstructed from the parity
data and user data on the remaining disks. Two disks must fail
before the entire array fails.
The read performance of a RAID 5 array is excellent—comparable to
that of a RAID 0 array. Write performance is lower than that of a
RAID 0 array, because write operations involve calculating and writ-
ing new parity data as well as writing the new user data.
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AHA-3980/3980W/3985/3985W User's Guide
Stock Number: 510810-00, Rev. D
Print Spec Number: 493479-00
Current Date: 3/1/99
Page: 2-2
ECN Date: 3/19/96