In Oracle Database 11g, such performance comparisons are made much easier. You
can preserve AWR baselines that contain performance data from specific time periods.
Baselines can be established for fixed times or moving windows, or they can
serve as a template.
Oracle and Disk I/O Resources
From the perspective of machine resources, an input/output operation, or I/O, can be
defined as the operating system of the computer reading or writing some bytes from or
to the underlying disk subsystem of the database server. I/Os can be small, such as 4
KB of data, or large, such as 64 KB or 128 KB of data. The lower and upper limits on
the size of an I/O operation vary according to the operating system. Your Oracle database
also has a block size that you can define, called the database block size.
An Oracle database issues I/O requests in two basic sizes:
Single database block I/Os
For example, one 8 KB datablock at a time. This type of request reads or writes a
specific block. For example, after looking up a row in an index, Oracle uses a
single block I/O to retrieve the desired database block.
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