Friday, October 21, 2005

Tuning the I/O Subsystem

Tuning the I/O Subsystem

I/O is probably one of the most common problems facing Oracle users.
In many cases, the performance of the system is entirely limited by
disk I/O. In some cases, the system actually becomes idle waiting for
disk requests to complete. We say that these systems are I/O bound or
disk bound.

As you see in Chapter 14, "Advanced Disk I/O Concepts," disks have
certain inherent limitations that cannot be overcome. Therefore, the
way to deal with disk I/O issues is to understand the limitations of
the disks and design your system with these limitations in mind.
Knowing the performance characteristics of your disks can help you in
the design stage.

Optimizing your system for I/O should happen during the design stage.
As you see in Part III, "Configuring the System," different types of
systems have different I/O patterns and require different I/O designs.
Once the system is built, you should first tune for memory and then
tune for disk I/O. The reason you tune in this order is to make sure
that you are not dealing with excessive cache misses, which cause
additional I/Os.

The strategy for tuning disk I/O is to keep all drives within their
physical limits. Doing so reduces queuing timeā€”and thus increases
performance. In your system, you may find that some disks process many
more I/Os per second than other disks. These disks are called "hot
spots." Try to reduce hot spots whenever possible. Hot spots occur
whenever there is a lot of contention on a single disk or set of
disks.
Understanding Disk Contention

Disk contention occurs whenever the physical limitations of a disk
drive are reached and other processes have to wait. Disk drives are
mechanical and have a physical limitation on both disk seeks per
second and throughput. If you exceed these limitations, you have no
choice but to wait.

You can find out if you are exceeding these limits both through
Oracle's file I/O statistics and through operating system statistics.
This chapter looks at the Oracle statistics; Chapter 12, "Operating
System-Specific Tuning," looks at the operating system statistics for
some popular systems.

Although the Oracle statistics give you an accurate picture of how
many I/Os have taken place for a particular data file, they may not
accurately represent the entire disk because other activity outside of
Oracle may be incurring disk I/Os. Remember that you must correlate
the Oracle data file to the physical disk on which it resides.

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