IBM 866251Y - Netfinity 5500 M20, Hardware Maintenance Manual

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Understanding RAID technology
RAID is the technology of grouping several hard disk
drives in a computer into an
array that can be defined as
one or more logical drives. Each logical drive appears to
the operating system as a single drive. This grouping
technique greatly enhances logical-drive capacity and
performance beyond the physical limitations of a single
hard disk drive.
When multiple physical hard disk drives are grouped into a
logical drive, the ServeRAID controller can transfer data in
parallel from the multiple drives in the array. This parallel
transfer yields data-transfer rates that are many times
higher than with nonarrayed drives. This increased speed
makes the system better able to meet the
throughput (the
amount of work in a given amount of time) or productivity
needs of the multiple-user network environment.
The ability to respond to multiple data requests provides
not only an impressive increase in throughput, but also a
decrease in response time. The combination of parallel
transfers and simultaneous responses to multiple requests
allows disk arrays to provide a high level of performance in
network environments.
Interleave depth and stripe-unit size:
With RAID technology, data is striped across an array of
hard disk drives. This data-distribution scheme
complements the way the operating system requests data.
The granularity at which data from one file is stored on one
drive of the array before subsequent data is stored on the
next drive of the array is called the
interleave depth.
You can control the interleave depth and maximize the
performance of a ServeRAID controller by setting a
stripe-unit size that is close to the size of the system I/O
requests. You can set the stripe-unit size to 8 KB, 16 KB,
32 KB, or 64 KB. For example, performance in
transaction-based environments, which typically involve
large blocks of data, might be optimal when the stripe-unit
size is set to 32 KB or 64 KB; however, performance in file
and print environments, which typically involve multiple
small blocks of data, might be optimal when the stripe-unit
size is set to 8 KB or 16 KB.
The collection, in logical order of these stripe units, from
the first drive of the array to the last drive of the array, is
called a
stripe. (See page 90 for more information.)
Supported RAID levels:
Disk arrays are used to improve performance and
reliability. The amount of improvement depends on the
application programs that are run on the server and the
RAID levels that are assigned to the logical drives.
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