Service manual

Sizing Disk Subsystems
Chapter 4 Hardware Sizing 91
RAID Alternatives
RAIDstandsfor Redundant Arrayof InexpensiveDisks. As the name suggests, the
primary purpose of RAID is to provide resiliency. Ifone disk in the array fails, data
on that disk is not lost but remains available on one or more other disks in the
array. To implement resiliency, RAID provides an abstraction allowing multiple
disk drives to be configured as a larger virtual disk, usually referred to as a
volume. This is achieved by concatenating, mirroring, or striping physical disks.
Concatenation is implemented by having blocks of one disk logically follow those
of another disk. For example, disk 1 has blocks 0-99, disk 2 has blocks 100-199 and
so forth. Mirroring is implemented by copying blocks of one disk to another and
then keeping them in continuous synchronization. Striping uses algorithms to
distribute virtual disk blocks over multiple physical disks.
The purpose of striping is performance. Random writes can be dealt with very
quickly as data being written is likely to be destined for more than one of the disks
in the striped volume, hence the disks are able to work in parallel. The same
applies to random reads. For large sequential reads and writes the case may not be
quite so clear. It has been observed, however, that sequential I/O performance can
be improved. An application generating many I/O requests can swamp a single
disk controller, for example. If the disks in the striped volume all have their own
dedicated controller, however, swamping is far less likely to occur and so
performance is improved.
RAID can be implemented using either a software or a hardware RAID manager
device. There are advantages and disadvantages in using either method:
• Hardware RAID generally provides higher performance as it is implemented
in hardware and hence incurs less processing overhead than software RAID.
Furthermore,hardware RAID is dissociated fromthehost system, leaving host
resources free to execute applications.
• Hardware RAID is generally more expensive than software RAID.
• Software RAID can be more flexible than hardware RAID. For example, a
hardware RAID manager is usually associated with a single array of disks or
with a prescribed set of arrays, whereas software RAID can encapsulateany
number of arrays of disks, or, if desired, only certain disks within an array.
The following sections discuss RAID configurations, known as levels. The most
common RAID levels, 0, 1, 1+0 and 5 are covered in some detail, whereas less
common levels are merely compared and contrasted.