Setting the DBIO_EXPECTED Parameter

ADDM analysis of I/O performance partially depends on a single argument, DBIO_EXPECTED, that describes the expected performance of the I/O subsystem. The value of DBIO_EXPECTED is the average time it takes to read a single database block, in microseconds. Oracle Database uses the default value of 10 milliseconds, which is an appropriate value for most hard drives. You can choose a different value based on the characteristics of your hardware.

To determine the correct setting for the DBIO_EXPECTED initialization parameter:

1. Measure the average read time of a single database block for your hardware.

   This measurement must be taken for random I/O, which includes seek time if you use standard hard drives. Typical values for hard drives are between 5000 and 20000 microseconds.

   See Also:

   Oracle Database Performance Tuning Guide to learn how to assess the I/O capability of the storage subsystem

2. Set the value one time for all subsequent ADDM executions.

   For example, if the measured value is 8000 microseconds, then execute the following PL/SQL code as the SYS user:

   EXECUTE DBMS_ADVISOR.SET_DEFAULT_TASK_PARAMETER(
                        'ADDM', 'DBIO_EXPECTED', 8000);