Monitoring System Performance
Monitoring system and database performance is a complex task, and there can be many aspects to managing performance, including memory, disk, CPU, database objects, and database user sessions—
just for starters. This chapter zeroes in on using Oracle’s Automatic Workload Repository (AWR) to gather data about the activities occurring within your database, and help convert that raw data into useful information to help gauge performance within your database for a specific period of time.
Usually, when there are performance issues occurring within a database, it’s easy to know when the performance problems are occurring because database activity is “slow” during that given time frame.
Knowing this time frame is the starting point to perform the analysis using the AWR information.
The AWR is created automatically when you create your Oracle database, and automatically gathers statistics on the activities occurring within your database. Some of this data is time or very near real-time, and some of the data represents historical statistics on these activities. The most current data on active sessions is stored in the Active Session History (ASH) component of the performance statistics repository. The more historical snapshots of data are generally known as the AWR snapshots.
The AWR process captures hourly snapshots by default from Oracle’s dynamic performance views, and stores them within the AWR. This gives the DBA the ability to view database activity over any period of time, whether it is a single-hour time frame, up to all activity that is stored within the AWR. For instance, if you have a period of time where your database is performing poorly, you can generate an AWR report that will give statistics on the database activity for only that specific period of time.
The ASH component of the AWR is really meant to give the DBA a more real-time look at session information that is not captured within the AWR snapshots. The session information stored within the ASH repository is data that is sampled every second from the dynamic performance views.
Oracle has had similar information within the database for many years with its predecessors UTLBSTAT/UTLESTAT and Statspack, but the report data hasn’t been generated or saved automatically until AWR came along with Oracle 10g. This information can now help monitor your database
performance much more easily, whether it be analyzing real-time data on activities currently going on within your database, or historical information that could be days, weeks, or even months old, depending on the configuration of the AWR within your database.
4-1. Implementing Automatic Workload Repository (AWR)
Problem
You want to store historical database performance statistics on your database for tuning purposes.
Solution
By implementing and using the Automatic Workload Repository (AWR) within your database, Oracle will store interval-based historical statistics in your database for future reference. This information can be used to see what was going on within your database within a given period of time. By default, the AWR should be enabled within your database. The key initialization parameter to validate is the
STATISTICS_LEVEL parameter:
SQL> show parameter statistics_level
NAME TYPE VALUE
--- --- --- statistics_level string TYPICAL
This parameter can be set to BASIC, TYPICAL (which is the default), and ALL. As long as the parameter is set to TYPICAL or ALL, statistics will be gathered for the AWR. If the parameter is set to BASIC, you simply need to modify the parameter in order to start gathering AWR statistics for your database:
alter system set statistics_level=TYPICAL scope=both;
How It Works
The predecessor of AWR, which is Statspack, requires manual setup and configuration to enable the statistics gathering. As stated, there generally is no setup required, unless the STATISTICS_LEVEL parameter has been changed to the BASIC setting. By default, an AWR snapshot is taken every hour on your database, and is stored, by default, for eight days. These are configurable settings that can be modified, if desired. See Recipe 4-2 for information on modifying the default settings of the AWR snapshots.
In addition to simply seeing the value of the STATISTICS_LEVEL parameter, you can also view the V$STATISTICS_LEVEL view to see this information, which has information on the STATISTICS_LEVEL setting, as well as all other relevant statistical components within your database:
SELECT statistics_name, activation_level, system_status
CHAPTER 4 ■ MONITORING SYSTEM PERFORMANCE
Undo Advisor, Alerts and Fast Ramp up TYPICAL ENABLED Streams Pool Advice TYPICAL ENABLED Time Model Events TYPICAL ENABLED Plan Execution Sampling TYPICAL ENABLED Automated Maintenance Tasks TYPICAL ENABLED SQL Monitoring TYPICAL ENABLED Adaptive Thresholds Enabled TYPICAL ENABLED V$IOSTAT_* statistics TYPICAL ENABLED Session Wait Stack TYPICAL ENABLED 24 rows selected.
The type of information that is stored in the AWR includes the following:
• Statistics regarding object access and usage
• Time model statistics
• System statistics
• Session statistics
• SQL statements
The information gathered is then grouped and formatted by category. Some of the categories found on the report include the following:
• Instance efficiency
• Top 5 timed events
• Memory and CPU statistics
• Wait information
• SQL statement information
• Miscellaneous operating system and database statistics
• Database file and tablespace usage information
■ Note To use AWR functionality, the following must apply. First, you must be licensed for the Oracle