User's Manual

34 Understanding RAID Concepts
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
system performance. You can, however, use software RAID on top of
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 at all times. 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.