Understanding Oracle physical Disk I/O metrics
 

We must remember that Oracle does not run in a vacuum, and that there are important "external" events that take place outside the scope of the Oracle instance.  We see this frequently with the Oracle disk I/O metrics.

I/O remains a critical part of Oracle performance tuning.  While everything else in Oracle runs in microseconds and RAM moves in nanoseconds, the mechanical disk devices on some Oracle servers become a major bottleneck.

When tuning Oracle, we often forget to look outside the instance, examining external influences on disk I/O speed.  These may include:

When Oracle makes a physical I/O request, it's handed-off to the operating system as a native I/O operation.  At this layer, the device-media interface takes over, as Oracle patiently waits for the block to be returned to the calling routine.  During this time, many external events can influence the observed I/O timings.

Read more about Oracle physical disk I/O metrics:

http://www.dba-oracle.com/t_physical_io_disk_metrics.htm

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