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Oracle Essentials: Oracle9i, Oracle8i and Oracle8, Second Edition by Jonathan Stern, Robert Stackowiak, Rick Greenwald

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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.

An Oracle database issues I/O requests in two basic sizes:

Single database block I/Os

For example, 8 KB 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.

Multiblock I/Os

For example, 32 database blocks, each consisting of 8 KB, for a total I/O size of 256 KB. Multiblock I/O is used for large-scale operations, such as full table scans. The number of blocks in one multiblock I/O is determined by the initialization parameter DB_FILE_MULTIBLOCK_READ_COUNT.

The Oracle database can read larger amounts of data with multiblock I/Os, so there are times when a full table scan might actually retrieve data faster than an index-based retrieval (e.g., if the selectivity of the index is low).

I/O Planning Principles for an Oracle Database

When you’re planning the disk layout and subsequent placement of the various files that make up your database, you need to consider the different reasons Oracle performs I/O and the potential ...

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