RMAN backup
and restore operations perform the following distinct tasks:
•
Reading
or writing input data
•
Processing
data by validating and copying blocks from the input to the output buffers
The slowest
of these tasks is called a bottleneck. RMAN tuning involves identifying
the bottleneck (or bottlenecks) and attempting to make it more efficient by
using RMAN commands, initialization parameter settings, or adjustments to the
physical media. The key to tuning RMAN is understanding input/output (I/O). The
backup and restore jobs of RMAN use two types of I/O buffers: disk and tertiary
storage (usually tape). When performing a backup, RMAN reads input files by
using disk buffers and writes the output backup file by using either the disk
or the tape buffer. When performing restores, RMAN reverses these roles. I/O
can be synchronous and asynchronous. Synchronous devices perform only one I/O
task at a time. Therefore, you can easily determine how much time the backup
jobs require. In contrast to synchronous I/O (SIO), asynchronous I/O (AIO) can
perform more than one task at a time. To tune RMAN effectively, you must
thoroughly understand the concepts of synchronous and asynchronous I/O, disk
and tape buffers, and channel architecture. With an understanding of these
concepts, you can use fixed views to monitor bottlenecks.
You may be
able to take advantage of some backup and recovery features that allow you to
balance the performance of backup operations versus recovery operations. For
example, if you require shorter recovery time, then you may want to perform
image copy recovery on a regular basis. That takes more resources to prepare
for recovery, but would lessen the amount of time needed to perform the
recovery.
