5 Automatic Performance Statistics

5.1.1.1 Wait Events

Wait events are statistics that are incremented by a server process/thread to indicate that it had to wait for an event to complete before being able to continue processing. Wait event data reveals various symptoms of problems that might be impacting performance, such as latch contention, buffer contention, and I/O contention.

To enable easier high-level analysis of the wait events, the events are grouped into classes. The wait event classes include: Administrative, Application, Cluster, Commit, Concurrency, Configuration, Idle, Network, Other, Scheduler, System I/O, and User I/O.

The wait classes are based on a common solution that usually applies to fixing a problem with the wait event. For example, exclusive TX locks are generally an application level issue and HW locks are generally a configuration issue.

The following list includes common examples of the waits in some of the classes:

• Application: locks waits caused by row level locking or explicit lock commands

• Commit: waits for redo log write confirmation after a commit

• Idle: wait events that signify the session is inactive, such as SQL*Net message from client

• Network: waits for data to be sent over the network

• User I/O: wait for blocks to be read off a disk

  See Also:

  Oracle Database Reference for more information about Oracle wait events

5.1.1.2 Time Model Statistics

When tuning an Oracle system, each component has its own set of statistics. To look at the system as a whole, it is necessary to have a common scale for comparisons. Because of this, most Oracle advisories and reports describe statistics in terms of time. In addition, the V$SESS_TIME_MODEL and V$SYS_TIME_MODEL views provide time model statistics. Using the common time instrumentation helps to identify quantitative effects on the database operations.

The most important of the time model statistics is DB time. This statistics represents the total time spent in database calls and is a indicator of the total instance workload. It is calculated by aggregating the CPU and wait times of all sessions not waiting on idle wait events (non-idle user sessions).

DB time is measured cumulatively from the time that the instance was started. Because DB time it is calculated by combining the times from all non-idle user sessions, it is possible that the DB time can exceed the actual time elapsed since the instance started up. For example, a instance that has been running for 30 minutes could have four active user sessions whose cumulative DB time is approximately 120 minutes.

The objective for tuning an Oracle system could be stated as reducing the time that users spend in performing some action on the database, or simply reducing DB time. Other time model statistics provide quantitative effects (in time) on specific actions, such as logon operations and hard and soft parses.

5.1.1.3 Active Session History (ASH)

The V$ACTIVE_SESSION_HISTORY view provides sampled session activity in the instance. Active sessions are sampled every second and are stored in a circular buffer in SGA. Any session that is connected to the database and is waiting for an event that does not belong to the Idle wait class is considered as an active session. This includes any session that was on the CPU at the time of sampling.

Each session sample is a set of rows and the V$ACTIVE_SESSION_HISTORY view returns one row for each active session per sample, returning the latest session sample rows first. Because the active session samples are stored in a circular buffer in SGA, the greater the system activity, the smaller the number of seconds of session activity that can be stored in the circular buffer. This means that the duration for which a session sample appears in the V$ view, or the number of seconds of session activity that is displayed in the V$ view, is completely dependent on the database activity.

As part of the Automatic Workload Repository (AWR) snapshots, the content of V$ACTIVE_SESSION_HISTORY is also flushed to disk. Because the content of this V$ view can get quite large during heavy system activity, only a portion of the session samples is written to disk.

By capturing only active sessions, a manageable set of data is represented with the size being directly related to the work being performed rather than the number of sessions allowed on the system. Using the Active Session History enables you to examine and perform detailed analysis on both current data in the V$ACTIVE_SESSION_HISTORY view and historical data in the DBA_HIST_ACTIVE_SESS_HISTORY view, often avoiding the need to replay the workload to gather additional performance tracing information. The data present in ASH can be rolled up on various dimensions that it captures, including the following:

• SQL identifier of SQL statement

• Object number, file number, and block number

• Wait event identifier and parameters

• Session identifier and session serial number

• Module and action name

• Client identifier of the session

• Service hash identifier

  See Also:

  Oracle Database Reference for more information about the V$ACTIVE_SESSION_HISTORY view

Active Session History information over a specified duration can be gathered into a report. For more information, see "Generating Active Session History Reports".

5.1.1.4 System and Session Statistics

A large number of cumulative database statistics are available on a system and session level through the V$SYSSTAT and V$SESSTAT views.

5.1.2.1 CPU Statistics

CPU utilization is the most important operating system statistic in the tuning process. Get CPU utilization for the entire system and for each individual CPU on multi-processor environments. Utilization for each CPU can detect single-threading and scalability issues.

Most operating systems report CPU usage as time spent in user space or mode and time spent in kernel space or mode. These additional statistics allow better analysis of what is actually being executed on the CPU.

On an Oracle data server system, where there is generally only one application running, the server runs database activity in user space. Activities required to service database requests (such as scheduling, synchronization, I/O, memory management, and process/thread creation and tear down) run in kernel mode. In a system where all CPU is fully utilized, a healthy Oracle system runs between 65% and 95% in user space.

The V$OSSTAT view captures system level information in the database, making it easier for you to determine if there are hardware level resource issues. The V$SYSMETRIC_HISTORY view shows a one-hour history of the Host CPU Utilization metric, a representation of percentage of CPU usage at each one-minute interval. The V$SYS_TIME_MODEL view supplies statistics on the CPU usage by the Oracle database. Using both sets of statistics enable you to determine whether the Oracle database or other system activity is the cause of the CPU problems.

5.1.2.2 Virtual Memory Statistics

Virtual memory statistics should mainly be used as a check to validate that there is very little paging or swapping activity on the system. System performance degrades rapidly and unpredictably when paging or swapping occurs.

Individual process memory statistics can detect memory leaks due to a programming failure to deallocate memory taken from the process heap. These statistics should be used to validate that memory usage does not increase after the system has reached a steady state after startup. This problem is particularly acute on shared server applications on middle tier systems where session state may persist across user interactions, and on completion state information that is not fully deallocated.

5.1.2.3 Disk Statistics

Because the database resides on a set of disks, the performance of the I/O subsystem is very important to the performance of the database. Most operating systems provide extensive statistics on disk performance. The most important disk statistics are the current response time and the length of the disk queues. These statistics show if the disk is performing optimally or if the disk is being overworked.

Measure the normal performance of the I/O system; typical values for a single block read range from 5 to 20 milliseconds, depending on the hardware used. If the hardware shows response times much higher than the normal performance value, then it is performing badly or is overworked. This is your bottleneck. If disk queues start to exceed two, then the disk is a potential bottleneck of the system.

5.1.2.4 Network Statistics

Network statistics can be used in much the same way as disk statistics to determine if a network or network interface is overloaded or not performing optimally. In today's networked applications, network latency can be a large portion of the actual user response time. For this reason, these statistics are a crucial debugging tool.

5.1.2.5 Operating System Data Gathering Tools

Table 5-1 shows the various tools for gathering operating statistics on UNIX. For Windows, use the Performance Monitor tool.

5.3.1.1 Creating Snapshots

You can manually create snapshots with the CREATE_SNAPSHOT procedure if you want to capture statistics at times different than those of the automatically generated snapshots. For example:

BEGIN
  DBMS_WORKLOAD_REPOSITORY.CREATE_SNAPSHOT ();
END;
/

In this example, a snapshot for the instance is created immediately with the flush level specified to the default flush level of TYPICAL. You can view this snapshot in the DBA_HIST_SNAPSHOT view.

5.3.1.2 Dropping Snapshots

You can drop a range of snapshots using the DROP_SNAPSHOT_RANGE procedure. To view a list of the snapshot Ids along with database Ids, check the DBA_HIST_SNAPSHOT view. For example, you can drop the following range of snapshots:

BEGIN
  DBMS_WORKLOAD_REPOSITORY.DROP_SNAPSHOT_RANGE (low_snap_id => 22, 
                           high_snap_id => 32, dbid => 3310949047);
END;
/

In the example, the range of snapshot Ids to drop is specified from 22 to 32. The optional database identifier is 3310949047. If you do not specify a value for dbid, the local database identifier is used as the default value.

Active Session History data (ASH) that belongs to the time period specified by the snapshot range is also purged when the DROP_SNAPSHOT_RANGE procedure is called.

5.3.1.3 Modifying Snapshot Settings

You can adjust the interval, retention, and captured Top SQL of snapshot generation for a specified database Id, but note that this can affect the precision of the Oracle diagnostic tools.

The INTERVAL setting affects how often in minutes that snapshots are automatically generated. The RETENTION setting affects how long in minutes that snapshots are stored in the workload repository. The TOPNSQL setting affects the number of Top SQL to flush for each SQL criteria (Elapsed Time, CPU Time, Parse Calls, Shareable Memory, and Version Count). The value for this setting will not be affected by the statistics/flush level and will override the system default behavior for the AWR SQL collection. It is possible to set the value for this setting to MAXIMUM to capture the complete set of SQL in the cursor cache, though by doing so (or by setting the value to a very high number) may lead to possible space and performance issues since there will more data to collect and store. To adjust the settings, use the MODIFY_SNAPSHOT_SETTINGS procedure. For example:

BEGIN
  DBMS_WORKLOAD_REPOSITORY.MODIFY_SNAPSHOT_SETTINGS( retention => 43200, 
                 interval => 30, topnsql => 100, dbid => 3310949047);
END;
/

In this example, the retention period is specified as 43200 minutes (30 days), the interval between each snapshot is specified as 30 minutes, and the number of Top SQL to flush for each SQL criteria as 100. If NULL is specified, the existing value is preserved. The optional database identifier is 3310949047. If you do not specify a value for dbid, the local database identifier is used as the default value. You can check the current settings for your database instance with the DBA_HIST_WR_CONTROL view.

5.3.2.1 Creating a Baseline

This section describes how to create a baseline using an existing range of snapshots.

To create a baseline:

1. Review the existing snapshots in the DBA_HIST_SNAPSHOT view to determine the range of snapshots that you want to use.

2. Use the CREATE_BASELINE procedure to create a baseline using the desired range of snapshots:

   BEGIN
       DBMS_WORKLOAD_REPOSITORY.CREATE_BASELINE (start_snap_id => 270, 
                      end_snap_id => 280, baseline_name => 'peak baseline', 
                      dbid => 3310949047, expiration => 30);
   END;
   /
   

   In this example, 270 is the start snapshot sequence number and 280 is the end snapshot sequence. The name of baseline is peak baseline. The optional database identifier is 3310949047. If you do not specify a value for dbid, the local database identifier is used as the default value. The optional expiration parameter is set to 30, so the baseline will expire and be dropped automatically after 30 days. If you do not specify a value for expiration, the baseline will never expire.

The system automatically assign a unique baseline Id to the new baseline when the baseline is created. The baseline Id and database identifier are displayed in the DBA_HIST_BASELINE view.

5.3.2.2 Dropping a Baseline

This section describes how to drop an existing baseline. Periodically, you may want to drop a baseline that is no longer used to conserve disk space. The snapshots associated with a baseline are retained indefinitely until you explicitly drop the baseline or the baseline has expired.

To drop a baseline:

1. Review the existing baselines in the DBA_HIST_BASELINE view to determine the baseline that you want to drop.

2. Use the DROP_BASELINE procedure to drop the desired baseline:

   BEGIN
     DBMS_WORKLOAD_REPOSITORY.DROP_BASELINE (baseline_name => 'peak baseline',
                     cascade => FALSE, dbid => 3310949047);
   END;
   /
   

   In the example, the name of baseline is peak baseline. The cascade parameter is set to FALSE, which specifies that only the baseline is dropped. Setting this parameter to TRUE specifies that the drop operation will also remove the snapshots associated with the baseline. The optional dbid parameter specifies the database identifier, which in this example is 3310949047. If you do not specify a value for dbid, the local database identifier is used as the default value.

5.3.3.1 Extracting AWR Data

The awrextr.sql script extracts the AWR data for a range of snapshots from the database into a Data Pump export file. Once created, this dump file can be transported to another system where the extracted data can be loaded. To run the awrextr.sql script, you need to be connected to the database as the SYS user.

To extract AWR data:

1. At the SQL prompt, enter:

   @$ORACLE_HOME/rdbms/admin/awrextr.sql
   

   A list of the databases in the AWR schema is displayed.

2. Specify the database from which the AWR data will be extracted:

   Enter value for db_id: 1377863381
   

   In this example, the database with the database identifier of 1377863381 is selected.

3. Specify the number of days for which you want to list snapshot Ids.

   Enter value for num_days: 2
   

   A list of existing snapshots for the specified time range is displayed. In this example, snapshots captured in the last 2 days are displayed.

4. Define the range of snapshots for which AWR data will be extracted by specifying a beginning and ending snapshot Id:

   Enter value for begin_snap: 30
   Enter value for end_snap: 40
   

   In this example, the snapshot with a snapshot Id of 30 is selected as the beginning snapshot, and the snapshot with a snapshot Id of 40 is selected as the ending snapshot.

5. A list of directory objects is displayed.

   Specify the directory object pointing to the directory where the export dump file will be stored:

   Enter value for directory_name: DATA_PUMP_DIR
   

   In this example, the directory object DATA_PUMP_DIR is selected.

6. Specify the prefix for name of the export dump file (the .dmp suffix will be automatically appended):

   Enter value for file_name: awrdata_30_40
   

   In this example, an export dump file named awrdata_30_40 will be created in the directory corresponding to the directory object you specified:

   Dump file set for SYS.SYS_EXPORT_TABLE_01 is:
   C:\ORACLE\PRODUCT\11.1.0.5\DB_1\RDBMS\LOG\AWRDATA_30_40.DMP
   Job "SYS"."SYS_EXPORT_TABLE_01" successfully completed at 08:58:20
   

   Depending on the amount of AWR data that needs to be extracted, the AWR extract operation may take a while to complete. Once the dump file is created, you can use Data Pump to transport the file to another system.

5.3.3.2 Loading AWR Data

Once the export dump file is transported to the target system, you can load the extracted AWR data using the awrload.sql script. The awrload.sql script will first create a staging schema where the snapshot data is transferred from the Data Pump file into the database. The data is then transferred from the staging schema into the appropriate AWR tables. To run the awrload.sql script, you need to be connected to the database as the SYS user.

To load AWR data:

1. At the SQL prompt, enter:

   @$ORACLE_HOME/rdbms/admin/awrload.sql
   

   A list of directory objects is displayed.

2. Specify the directory object pointing to the directory where the export dump file is located:

   Enter value for directory_name: DATA_PUMP_DIR
   

   In this example, the directory object DATA_PUMP_DIR is selected.

3. Specify the prefix for name of the export dump file (the .dmp suffix will be automatically appended):

   Enter value for file_name: awrdata_30_40
   

   In this example, the export dump file named awrdata_30_40 is selected.

4. Specify the name of the staging schema where the AWR data will be loaded:

   Enter value for schema_name: AWR_STAGE
   

   In this example, a staging schema named AWR_STAGE will be created where the AWR data will be loaded.

5. Specify the default tablespace for the staging schema:

   Enter value for default_tablespace: SYSAUX
   

   In this example, the SYSAUX tablespace is selected.

6. Specify the temporary tablespace for the staging schema:

   Enter value for temporary_tablespace: TEMP
   

   In this example, the TEMP tablespace is selected.

7. A staging schema named AWR_STAGE will be created where the AWR data will be loaded. After the AWR data is loaded into the AWR_STAGE schema, the data will be transferred into the AWR tables in the SYS schema:

   Processing object type TABLE_EXPORT/TABLE/CONSTRAINT/CONSTRAINT
   Completed 113 CONSTRAINT objects in 11 seconds
   Processing object type TABLE_EXPORT/TABLE/CONSTRAINT/REF_CONSTRAINT
   Completed 1 REF_CONSTRAINT objects in 1 seconds
   Job "SYS"."SYS_IMPORT_FULL_03" successfully completed at 09:29:30
   ... Dropping AWR_STAGE user
   End of AWR Load
   

   Depending on the amount of AWR data that needs to be loaded, the AWR load operation may take a while to complete. After the AWR data is loaded, the staging schema will be dropped automatically.

5.3.5.1 Running the awrrpt.sql Report

To generate an HTML or text report for a range of snapshot Ids, run the awrrpt.sql script at the SQL prompt:

@$ORACLE_HOME/rdbms/admin/awrrpt.sql

First, you need to specify whether you want an HTML or a text report.

Enter value for report_type: text

Specify the number of days for which you want to list snapshot Ids.

Enter value for num_days: 2

After the list displays, you are prompted for the beginning and ending snapshot Id for the workload repository report.

Enter value for begin_snap: 150
Enter value for end_snap: 160

Next, accept the default report name or enter a report name. The default name is accepted in the following example:

Enter value for report_name: 
Using the report name awrrpt_1_150_160

The workload repository report is generated.

5.3.5.2 Running the awrrpti.sql Report

To specify a database and instance before entering a range of snapshot Ids, run the awrrpti.sql script at the SQL prompt to generate an HTML or text report:

@$ORACLE_HOME/rdbms/admin/awrrpti.sql

First, specify whether you want an HTML or a text report. After that, a list of the database identifiers and instance numbers displays, similar to the following:

Instances in this Workload Repository schema
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   DB Id    Inst Num DB Name      Instance     Host
----------- -------- ------------ ------------ ------------
 3309173529        1 MAIN         main         dlsun1690
 3309173529        1 TINT251      tint251      stint251

Enter the values for the database identifier (dbid) and instance number (inst_num) at the prompts.

Enter value for dbid: 3309173529
Using 3309173529 for database Id
Enter value for inst_num: 1

Next you are prompted for the number of days and snapshot Ids, similar to the awrrpt.sql script, before the text report is generated. See "Running the awrrpt.sql Report".

5.3.5.3 Running the awrsqrpt.sql Report

To generate an HTML or text report for a particular SQL statement, run the awrsqrpt.sql script at the SQL prompt:

@$ORACLE_HOME/rdbms/admin/awrsqrpt.sql

First, you need to specify whether you want an HTML or a text report.

Enter value for report_type: text

Specify the number of days for which you want to list snapshot Ids.

Enter value for num_days: 1

After the list displays, you are prompted for the beginning and ending snapshot Id for the workload repository report.

Enter value for begin_snap: 146
Enter value for end_snap: 147

Specify the SQL Id of a particular SQL statement to display statistics.

Enter value for sql_id: 2b064ybzkwf1y

Next, accept the default report name or enter a report name. The default name is accepted in the following example:

Enter value for report_name: 
Using the report name awrsqlrpt_1_146_147.txt

The workload repository report is generated.

5.3.5.4 Running the awrsqrpi.sql Report

To specify a database and instance before entering a particular SQL statement Id, run the awrsqrpi.sql script at the SQL prompt to generate an HTML or text report:

@$ORACLE_HOME/rdbms/admin/awrsqrpi.sql

First, you need to specify whether you want an HTML or a text report.

Enter value for report_type: text

Next, a list of the database identifiers and instance numbers displays, similar to the following:

Instances in this Workload Repository schema
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   DB Id    Inst Num DB Name      Instance     Host
----------- -------- ------------ ------------ ------------
 3309173529        1 MAIN         main         dlsun1690
 3309173529        1 TINT251      tint251      stint251

Enter the values for the database identifier (dbid) and instance number (inst_num) at the prompts.

Enter value for dbid: 3309173529
Using 3309173529 for database Id
Enter value for inst_num: 1
Using 1 for instance number

Next you are prompted for the number of days, snapshot Ids, SQL Id and report name, similar to the awrsqrpt.sql script, before the text report is generated. See "Running the awrsqrpt.sql Report".

5.3.5.5 Running the awrddrpt.sql Report

To compare detailed performance attributes and configuration settings between two time periods, run the awrddrpt.sql script at the SQL prompt to generate an HTML or text report:

@$ORACLE_HOME/rdbms/admin/awrddrpt.sql

First, you need to specify whether you want an HTML or a text report.

Enter value for report_type: text

Specify the number of days for which you want to list snapshot Ids for the first time period.

Enter value for num_days: 2

After the list displays, you are prompted for the beginning and ending snapshot Id for the first time period.

Enter value for begin_snap: 102
Enter value for end_snap: 103

Next, specify the number of days for which you want to list snapshot Ids for the second time period.

Enter value for num_days2: 1

After the list displays, you are prompted for the beginning and ending snapshot Id for the second time period.

Enter value for begin_snap2: 126
Enter value for end_snap2: 127

Next, accept the default report name or enter a report name. The default name is accepted in the following example:

Enter value for report_name: 
Using the report name awrdiff_1_102_1_126.txt

The workload repository report is generated.

5.3.5.6 Running the awrddrpi.sql Report

To specify a database and instance before selecting time periods to compare, run the awrddrpi.sql script at the SQL prompt to generate an HTML or text report:

@$ORACLE_HOME/rdbms/admin/awrddrpi.sql

First, you need to specify whether you want an HTML or a text report.

Enter value for report_type: text

Next, a list of the database identifiers and instance numbers displays, similar to the following:

Instances in this Workload Repository schema
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   DB Id    Inst Num DB Name      Instance     Host
----------- -------- ------------ ------------ ------------
 3309173529        1 MAIN         main         dlsun1690
 3309173529        1 TINT251      tint251      stint251

Enter the values for the database identifier (dbid) and instance number (inst_num) for the first time period at the prompts.

Enter value for dbid: 3309173529
Using 3309173529 for Database Id for the first pair of snapshots
Enter value for inst_num: 1
Using 1 for Instance Number for the first pair of snapshots

Specify the number of days for which you want to list snapshot Ids for the first time period.

Enter value for num_days: 2

After the list displays, you are prompted for the beginning and ending snapshot Id for the first time period.

Enter value for begin_snap: 102
Enter value for end_snap: 103

Next, enter the values for the database identifier (dbid) and instance number (inst_num) for the second time period at the prompts.

Enter value for dbid2: 3309173529
Using 3309173529 for Database Id for the second pair of snapshots
Enter value for inst_num2: 1
Using 1 for Instance Number for the second pair of snapshots

Specify the number of days for which you want to list snapshot Ids for the second time period.

Enter value for num_days2: 1

After the list displays, you are prompted for the beginning and ending snapshot Id for the second time period.

Enter value for begin_snap2: 126
Enter value for end_snap2: 127

Next, accept the default report name or enter a report name. The default name is accepted in the following example:

Enter value for report_name: 
Using the report name awrdiff_1_102_1_126.txt

The workload repository report is generated.

5.3.6.1 Running the ashrpt.sql Report

To generate a text report of ASH information, run the ashrpt.sql script at the SQL prompt:

@$ORACLE_HOME/rdbms/admin/ashrpt.sql

First, you need to specify whether you want an HTML or a text report.

Enter value for report_type: text

Specify the time frame to collect ASH information by first specifying the begin time in minutes prior to the system date.

Enter value for begin_time: -10

Next, enter the duration in minutes that the report for which you want to capture ASH information from the begin time. The default duration of system date minus begin time is accepted in the following example:

Enter value for duration:

The report in this example will gather ASH information beginning from 10 minutes before the current time and ending at the current time. Next, accept the default report name or enter a report name. The default name is accepted in the following example:

Enter value for report_name: 
Using the report name ashrpt_1_0310_0131.txt

The session history report is generated.

5.3.6.2 Running the ashrpti.sql Report

If you want to specify a database and instance before setting the time frame to collect ASH information, run the ashrpti.sql report at the SQL prompt to generate a text report:

@$ORACLE_HOME/rdbms/admin/ashrpti.sql

First, specify whether you want an HTML or a text report. After that, a list of the database Ids and instance numbers displays, similar to the following:

Instances in this Workload Repository schema
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   DB Id    Inst Num DB Name      Instance     Host
----------- -------- ------------ ------------ ------------
 3309173529        1 MAIN         main         dlsun1690
 3309173529        1 TINT251      tint251      stint251

Enter the values for the database identifier (dbid) and instance number (inst_num) at the prompts.

Enter value for dbid: 3309173529
Using 3309173529 for database id
Enter value for inst_num: 1

Next you are prompted for the begin time and duration to capture ASH information, similar to the ashrpt.sql script, before the report is generated. See "Running the ashrpt.sql Report".