Oracle 11g I/O calibration overview

Every time an Oracle ® Database reads or writes data to a disk, the database generates disk input and output (I/O) operations. The performance of many software applications is limited by disk I/O, and applications that spend the majority of central processing unit (CPU) time waiting for I/O activity to complete are I/O bound. I/O calibration helps to address this issue.

Introduction

Establishing a solid I/O subsystem is an essential part of building the infrastructure for an application. If any component in the I/O stack has limited throughput, it becomes the weakest link in the I/O flow. Because real-world workloads cannot be easily reproduced, validation of the I/O subsystem has always been a daunting task.

The I/O calibration feature of Oracle Database enables you to assess the performance of the storage subsystem and to determine whether I/O performance problems are caused by the database or by the storage subsystem. Unlike other external I/O calibration tools that issue I/O operations sequentially, the I/O calibration feature of Oracle Database issues I/O operations randomly by using Oracle data files to access the storage media. This produces results that more closely match the actual performance of the database.

The Calibrate I/O feature is based on a PL/SQL function called DBMS_RESOURCE_MANAGER.CALIBRATE_IO(). When Calibrate I/O is invoked, it generates workloads with the following characteristics:

• I/O intensive, read-only, random I/O (db_block_size)
• large-block (1 MB) sequential I/O workloads

The results from Calibrate I/O should be monitored and compared to the expected throughput rate (the maximum overall throughput of the I/O subsystem). I/O calibration can be used to evaluate the performance of the storage subsystem and to determine whether I/O performance problems result from the database host or the storage subsystem.

This blog covers the following topics:

• Prerequisites for I/O calibration
• Running I/O calibration
• Calibration considerations

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.

With synchronous I/O, when an I/O request is submitted to the operating system, the write process is blocked until the operation is completed. With asynchronous I/O, the calling process continues its work without blocking other requests that are still executing.

Ensure that asynchronous I/O is enabled for data files by running the following SQL 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';

The following image shows the query and results:

Note: Only one calibration can be performed on a database instance at a time.

Running I/O calibration

The I/O calibration feature of Oracle Database is accessed using the DBMS_RESOURCE_MANAGER.CALIBRATE_IO procedure. This procedure issues an I/O intensive read-only workload (made up of 1 MB of random of I/O operations) to the database files to determine the maximum IOPS (I/O requests per second) and MBPS (megabytes of I/O per second) that can be sustained by the storage subsystem.

The I/O calibration with the DBMS_RESOURCE_MANAGER.CALIBRATE_IO procedure includes the following steps:

1. The procedure issues random database-block-sized reads (by default, 8 KB) to all data files from all database instances. This step provides the maximum I/O operation per second (IOPS) in the output parameter (max_iops) that the database can sustain. The value of max_iops is an important metric for online transaction processing (OLTP) databases. The output parameter actual_latency provides the average latency for this workload. You can specify the specific target latency that you need with the input parameter max_latency, which specifies the maximum tolerable latency in milliseconds for database-block-sized I/O requests.

2. The procedure issues random 1 MB reads to all data files from all database instances. This step yields the output parameter max_mbps, which specifies the maximum megabytes per second (MBPS) of I/O that the database can sustain. This is an important metric for data warehouses.

The calibration runs more efficiently if the user provides the num_physical_disks input parameter, which specifies the approximate number of physical disks in the database storage system.

Because of the overhead from running the I/O workload, I/O calibration should only be performed when the database is idle, or during off-peak hours, to minimize the impact of the I/O calibration workload on the normal database workload.

The following image shows the output from a DBMS_RESOURCE_MANAGER.CALIBRATE_IO procedure:

Calibration considerations

Before running an I/O calibration, think about the following considerations:

• Run only 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 fails.
• Quiesce the database to minimize I/O on the instance.
• For Oracle Real Application Clusters (RAC) configurations, ensure that all instances are opened to calibrate the storage subsystem across nodes.
• For an Oracle RAC database, the workload is simultaneously generated from all instances.
• The num_physical_disks input parameter is optional. By setting this parameter to the approximate number of physical disks in the database’s storage system, the calibration is both faster and more accurate.

At any time during the I/O calibration process, you can query the calibration status in the V$IO_CALIBRATION_STATUS view. After I/O calibration is successfully completed, view the results in the DBA_RSRC_IO_CALIBRATE table, similar to the following example:

Conclusion

The I/O Resource Management (IORM) and Calibrate I/O features are valuable for understanding limitations of your current I/O architecture. After the calibration is completed, the information can be used to perform appropriate I/O design and sizing. Use the Feedback tab to make any comments or ask questions.