IBM ServeRAID, User Reference

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Chapter 3. Understanding RAID technology
Redundant array of independent disks (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 data processed 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 increase in
throughput, but also a decrease in response time. The combination of parallel
transfers and simultaneous responses to multiple requests enables disk arrays to
provide a high level of performance in network environments.
Note: If you already understand these concepts, go to Chapter 4, “Configuring
ServeRAID controllers” on page 35.
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 set the stripe-unit size to 8 KB, 16 KB, 32 KB, or 64 KB. You can
maximize the performance of your ServeRAID controller by setting the stripe-unit
size to a value that is close to the size of the system I/O requests. 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 of stripe units, from the first drive of the array to the last drive of the
array, is called a
stripe.
© Copyright IBM Corp. 2002
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