IBM ServeRAID-4 Ultra160 Series, User Reference

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Chapter 3. Understanding RAID technology
Redundant array of independent disk (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 allows 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 27.
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 of stripe units, from the first drive of the array to the last drive of the
array, is called a stripe.
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, 1, 1E, 5, 5E, 00, 10, 1E0, and 50.
© Copyright IBM Corp. 2001
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