254.Which three statements correctly describe the features of the I/O calibration process? (Choose
three.)
A. Only one I/O calibration process can run at a time.
B. It automates the resource allocation for the Automated Maintenance Tasks.
C. It improves the performance of the performance-critical sessions while running.
D. It can be used to estimate the maximum number of I/Os and maximum latency time for the system.
E. The latency time is computed only when the TIMED_STATISTICS initialization parameter is set to
TRUE.
Answer: ADE



http://docs.oracle.com/cd/E25178_01/server.1111/e16638/iodesign.htm

E正确：

8.3.1 Prerequisites for I/O Calibration

Before running I/O calibration, ensure that the following requirements are met:

• The user must be granted the SYSDBA privilege

• timed_statistics must be set to TRUE

• Asynchronous I/O must be enabled

  When using file systems, asynchronous I/O can be enabled by setting the FILESYSTEMIO_OPTIONS initialization parameter to SETALL.

• Ensure that asynchronous I/O is enabled for data files by running the following query:

  COL NAME FORMAT A50
  SELECT NAME,ASYNCH_IO FROM V$DATAFILE F,V$IOSTAT_FILE I
  WHERE  F.FILE#=I.FILE_NO
  AND    FILETYPE_NAME='Data File';
  

Additionally, only one calibration can be performed on a database instance at a time.

A正确：

When running the DBMS_RESOURCE_MANAGER.CALIBRATE_IO procedure, consider the following:

• Only run one calibration at a time on databases that use the same storage subsystem. If you simultaneously run the calibration across separate databases that use the same storage subsystem, the calibration will fail.

• Quiesce the database to minimize I/O on the instance.

• For Oracle Real Application Clusters (Oracle RAC) configurations, ensure that all instances are opened to calibrate the storage subsystem across nodes.

• For an Oracle Real Application Clusters (Oracle RAC) database, the workload is simultaneously generated from all instances.

• The num_physical_disks input parameter is optional. By setting the num_physical_disks parameter to the approximate number of physical disks in the database's storage system, the calibration can be faster and more accurate.

• In some cases, asynchronous I/O is permitted for data files, but the I/O subsystem for submitting asynchronous I/O may be maximized, and I/O calibration cannot continue. In such cases, refer to the port-specific documentation for information about checking the maximum limit for asynchronous I/O on the system.