Silicon Image SATA RAID FOR SILICON IMAGE, Manual

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SATARaid User’s Manual Copyright © 2003 Silicon Image Inc.
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2 An Introduction to RAID
RAID - Redundant Array of Independent Disks
RAID technology manages multiple disk drives to enhance I/O performance and provide redundancy i n order to withstand
the failure of any individual member, without the loss of data. There are many different methods of implementation for RAID,
with each having advantages and disadvantages. Raid levels or set types are given a numerical designator that defines its
implementation such as RAID 0 or RAID 1. SATARaid provides support for three RAID Set types: Striped (RAID 0) , Mirrored
(RAID 1), and RAID 10 Mirrored/Striped. Other RAID types are not supported by SATARaid software and thus are not
discussed.
Disk Striping (RAID 0)
Striping is a performance-oriented, non-redundant data mapping technique. While Striping is discussed as a RAID Set
type, it is does not provide any fault tolerance. With modern SATA and ATA bus mastering technology, multiple I/O
operations can be performed in parallel, enhancing data throughput. Striping arrays use multiple disks to form a larger
virtual disk. The figure below illustrates a three-disk stripe set. Stripe one is written to disk one, stripe two to disk two, and
so forth. RAID 0 sets can be comprised of two, three, or four drives.
Strip 2
Strip 5
Strip 8
Strip 11
Strip 0
Strip 3
Strip 6
Strip 9
Strip 1
Strip 4
Strip 7
Strip 10
Disk Mirroring (RAID 1)
Disk mirroring creates an identical twin for a selected disk by having the data simultaneously written to two disks. This
redundancy provides instantaneous protection from a single disk failure. If a read failure occurs on one drive, the system
reads the data from the other drive. RAID 1 sets are comprised of two drives. A third drive can be allocated as a spare in
case one of the drives in the set fails.
Block 0
Block 1
Block 2
Block 3
Block 0
Block 1
Block 2
Block 3
Disk Mirroring and Striping (RAID 10)
RAID 10 combines the features of both RAID 0 and RAID 1. Performance is provided through the use of Striping (RAID 0),
while adding the fault tolerance of Mirroring (RAID 1). The implementation of RAID 10 requires four drives. Th e drives are
assigned as two sets of striped pairs.
The data is written to RAID Set A, which is striped (RAID 0). This allows maximum speed. The data is then mirrored to
another RAID 0 striped set, which is Set B in the figure above. This provides data redundancy (RAID 1), and thus increased
data security.
Under certain circumstances, a RAID 10 set can sustain multiple simultaneous drive failures.