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. Thevalue 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 toassess 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);
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