StorCase Technology Fibre-to-SCSI Single RAID - Manual (Page 142)

130 Appendix B - Array Basics
StorCase Technology, Inc. S10C100 User's Guide - Rev. A01
RAID 1, 0/1, and 10 (Mirrored Disks)
In RAID 1, RAID 0/1, and RAID 10 arrays (commonly referred to as mirrored arrays), disks are
paired, with both disks in the 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 greater than that of a single disk, while
the write performance is slightly lower. In RAID 1/10 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 the pair must fail for the array
to fail. For example, an array of five pairs can still maintain 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 cost.
Since all disks must have a twin, twice the number of disks must be used in the array capacity
(an eight-disk array contains only four disks of usable capacity).
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 check disk).
In the even of a disk failure, the data can be reconstructed from corresponding data stripes
on the remaining disk 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 4
RAID 4 is similar to RAID 3 in that redundant information is achieved in the form of parity data.
The user data is distributed across all but one of the disks and uses a single, dedicated parity
disk for data protection. The main difference is that RAID 3 usually synchronizes writes to
its disks, while RAID 4 can send data to its disk independently.
RAID 4 is best suited for transaction processing applications that require high speed read
requests. RAID 4 is not recommended for I/O intensive applications that require high speed
data transfer rates.