Understanding Raid Technology; Stripe-Unit Size; Supported Raid Levels - IBM Netfinity ServeRAID-3H Installation And User Manual

Understanding RAID technology

RAID is the technology of grouping several physical drives in a computer into an array that you can define
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 physical drive.
When you group multiple physical drives 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.

Stripe-unit size

With RAID technology, data is striped across an array of physical drives. This data-distribution scheme
complements the way the operating system requests data.
The granularity at which data is stored on one drive of the array before subsequent data is stored on the
next drive of the array is called the stripe-unit size.
You can control the stripe-unit size and maximize the performance of your ServeRAID controller by setting
a stripe-unit size to a value 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.
Note: If you are using the ServeRAID-3L:
Eight physical drives are supported in an array if the stripe-unit size is set to 32 KB or 64 KB.
Sixteen physical drives are supported in an array if the stripe-unit size is set to 8 KB or 16 KB.
If you are using the ServeRAID-3H or ServeRAID-3HB, sixteen physical drives are supported in an
array if the stripe-unit size is set to 32 KB or 64 KB.

Supported RAID levels

Disk arrays are used to improve performance and reliability. The amount of improvement depends on the
application programs that you run on the server and the RAID levels that you assign to the logical drives.
The ServeRAID controllers support RAID level-0, RAID level-1, RAID level-1 Enhanced (1E), RAID level-5
and RAID level-5 Enhanced (5E).
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IBM ServeRAID-3H, ServeRAID-3HB, and ServeRAID-3L, Controllers