摘自2days performance tuning (e10822.dbf)

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 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);

meanwhile：see the value

SELECT * FROM DBA_ADVISOR_DEF_PARAMETERS WHERE PARAMETER_NAME='DBIO_EXPECTED';

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