| |
RAID is an acronym for Redundant Array of Independent Disks. It is an array of
multiple independent hard disk drives that provide high performance and fault
tolerance. The RAID subsystem controller implements several levels of the
Berkeley RAID technology. An appropriate RAID level is selected when the
volume sets are defined or created. This decision is based on disk capacity,
data availability (fault tolerance or redundancy), and disk performance. The
following is the RAID level, which support in the RAID subsystem.
The RAID subsystem controller makes the RAID implementation and the disks’
physical configuration transparent to the host operating system. This means
that the host operating system drivers and software utilities are not affected,
regardless of the RAID level selected. Correct installation of the disk array and
the controller requires a proper understanding of RAID technology and the
concepts.
Raid 0
RAID 0, also referred to as striping, writes stripping of data across multiple disk
drives instead of just one disk drive. RAID 0 does not provide any data
redundancy, but does offer the best high-speed data throughput. RAID 0 breaks
up data into smaller blocks and then writes a block to each drive in the array.
Disk striping enhances performance because multiple drives are accessed
simultaneously; but the reliability of RAID Level 0 is less than any of its member
disk drives due to its lack of redundancy.
Raid 1
RAID 1 also known as “disk mirroring”, data written to one disk drive is
simultaneously written to another disk drive. Read performance may be
enhanced if the array controller can parallel accesses both members of a
mirrored pair. During writes, there will be a minor performance penalty when
compared to writing to a single disk. If one drive fails, all data (and software
applications) are preserved on the other drive. RAID 1 offers extremely high
data reliability, but at the cost of doubling the required data storage capacity.
Raid 0+1
RAID 0+1 is a combination of RAID 0 and RAID 1, combing stripping with disk mirroring.
RAID Level 0+1 combines the fast performance of Level 0 with the data redundancy of
Leve1 1. In this configuration, data is distributed across several disk drives, similar to
Level 0, which are then duplicated to another set of drive for data protection. RAID 0+1
provides the highest read/write performance of any of the Hybrid RAID levels, but at the
cost of doubling the required data storage capacity.
Raid 3
RAID 3 provides disk striping and complete data redundancy through a
dedicated parity drive. RAID 3 breaks up data into smaller blocks, calculates
parity by performing an exclusive-or on the blocks, and then writes the blocks to
all but one drive in the array. The parity data created during the exclusive-or is
then written to the last drive in the array. If a single drive fails, data is still
available by computing the exclusive-or of the contents corresponding strips of
the surviving member disk. RAID-3 is best for applications that require very fast
data- transfer rates or long data blocks.
Raid 5
RAID 5 is sometimes called striping with parity at block level. In RAID 5, the
parity information is written to all of the drives in the subsystems rather than
concentrated on a dedicated parity disk. If one drive in the system fails, the
parity information can be used to reconstruct the data from that drive. All drives
in the array system can be used to seek operation at the same time, greatly
increasing the performance of the RAID system.
Raid 6
A RAID 6 array is essentially an extension of a RAID 5 array with a second
independent distributed parity scheme. Data and parity are striped on a block
level across multiple array members, just like in RAID 5, and a second set of
parity is calculated and written across all the drives. As larger disk arrays are
considered, it is desirable to use stronger codes that can tolerate multiple disk
failure. When a disk fails in a parity protected disk array, recovering the contents
of the failed disk requires successfully reading the contents of all no-failed disks.
RAID 6 provides an extremely high fault tolerance, and can sustain two
simultaneous drive failures without downtime or data loss.
Summary of RAID Levels
RAID subsystem supports RAID Levels 0, 1(0+1), 3, 5 and 6. The following
table provides a summary of RAID levels.
|
|
|
|
|
|
