Users Guide

Hardware and software RAID
RAID can be implemented with either hardware or software. A system using hardware RAID has a RAID controller that
implements the RAID levels and processes data reads and writes to the physical disks. When using software RAID provided
by the operating system, the operating system implements the RAID levels. For this reason, using software RAID by itself can
slow the system performance. You can, however, use software RAID along with hardware RAID volumes to provide better
performance and variety in the configuration of RAID volumes. For example, you can mirror a pair of hardware RAID 5 volumes
across two RAID controllers to provide RAID controller redundancy.
RAID concepts
RAID uses particular techniques for writing data to disks. These techniques enable RAID to provide data redundancy or better
performance. These techniques include:
● Mirroring — Duplicating data from one physical disk to another physical disk. Mirroring provides data redundancy by
maintaining two copies of the same data on different physical disks. If one of the disks in the mirror fails, the system can
continue to operate using the unaffected disk. Both sides of the mirror contain the same data always. Either side of the
mirror can act as the operational side. A mirrored RAID disk group is comparable in performance to a RAID 5 disk group in
read operations but faster in write operations.
● Striping — Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of consecutive virtual
disk data addresses that are mapped in fixed-size units to each physical disk in the virtual disk using a sequential pattern.
For example, if the virtual disk includes five physical disks, the stripe writes data to physical disks one through five without
repeating any of the physical disks. The amount of space consumed by a stripe is the same on each physical disk. The portion
of a stripe that resides on a physical disk is a stripe element. Striping by itself does not provide data redundancy. Striping in
combination with parity does provide data redundancy.
● Stripe size — The total disk space consumed by a stripe not including a parity disk. For example, consider a stripe that
contains 64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe size is 64KB and
the stripe element size is 16KB.
● Stripe element — A stripe element is the portion of a stripe that resides on a single physical disk.
● Stripe element size — The amount of disk space consumed by a stripe element. For example, consider a stripe that contains
64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe element size is 16KB and
the stripe size is 64KB.
● Parity — Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of the
striped disks fails, the data can be reconstructed from the parity information using the algorithm.
● Span — A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10, 50, or 60
virtual disk.
RAID levels
Each RAID level uses some combination of mirroring, striping, and parity to provide data redundancy or improved read and write
performance. For specific information on each RAID level, see Choosing raid levels.
Organizing data storage for availability and performance
RAID provides different methods or RAID levels for organizing the disk storage. Some RAID levels maintain redundant data so
that you can restore data after a disk failure. Different RAID levels also entail an increase or decrease in the I/O (read and write)
performance of a system.
Maintaining redundant data requires the use of additional physical disks. The possibility of a disk failure increases with an
increase in the number of disks. Since the differences in I/O performance and redundancy, one RAID level may be more
appropriate than another based on the applications in the operating environment and the nature of the data being stored.
When choosing a RAID level, the following performance and cost considerations apply:
● Availability or fault-tolerance — Availability or fault-tolerance refers to the ability of a system to maintain operations
and provide access to data even when one of its components has failed. In RAID volumes, availability or fault-tolerance
is achieved by maintaining redundant data. Redundant data includes mirrors (duplicate data) and parity information
(reconstructing data using an algorithm).
● Performance — Read and write performance can be increased or decreased depending on the RAID level you choose. Some
RAID levels may be more appropriate for particular applications.
192
Managing storage devices