Question: I wan to modify the frequency that I take my AWR snapshots. I understand that there is a utility that allows me to modify my AWR automatic snapshot settings. Can you please advise me on how to modify this snapshot settings?
Answer: The dba_hist_wr_control table shows the AWR snapshot settings, namely the snapshot interval, the retention period and the top-x-sql collection threshold:
SQL> desc
dba_hist_wr_control
Name
Null? Type
-------------------------------
-------- ----------------------------
DBID
NOT NULL NUMBER
SNAP_INTERVAL NOT
NULL INTERVAL DAY(5) TO SECOND(1)
RETENTION
NOT NULL INTERVAL DAY(5) TO SECOND(1)
TOPNSQL
VARCHAR2(10)
Also see my related notes on dba_hist_wr_control.
Scheduling the AWR snapshot collection
Oracle uses the MMON background process to collect AWR statistics.
You can disable
this job by changing the parameter STATISTICS_LEVEL to BASIC then snapshots will not be taken
automatically.
SQL> show
parameter statistics_level
NAME TYPE VALUE
------------------------------------ -----------
-------------------
statistics_level
string BASIC
You can re-enable this job by changing the parameter STATISTICS_LEVEL to TYPICAL then snapshots will be taken automatically.
AWR Snapshot Reports
Oracle provides reports that you can run to analyze the data in the AWR. There are two reports: awrrpt.sql and awrrpti.sql, which are available in the directory $ORACLE_HOME/rdbms/admin.
The output of these reports is essentially the same, except that awrrpti.sql script allows you to define a specific instance to report on.
The reports are much like the statspack reports of old, in that you define a beginning and ending snapshot ID, and the output filename of the report. Additionally, you can opt to produce the report in either text format or HTML format.
AWR Retention Period Settings
Setting an appropriate retention interval for your AWR is critical for proper data retention, especially for predictive modeling. You can adjust the AWR retention period according to your analysis needs.
In this example the retention period is specified as 3 years (1,576,800 minutes) and the interval between each snapshot is 60 minutes.
execute dbms_workload_repository.modify_snapshot_settings (
interval => 60,
retention => 1576800);
Oracle has published suggested storage requirements for AWR based on the activity and complexity of your 10g database:
| Active Sessions |
number of CPU's (num_cpus) |
number of schema objects |
Monthly space required (gig) |
|
| Small | 10 | 2 | 500 | 2 |
| Medium | 20 | 8 | 5,000 | 8 |
| Large | 100 | 32 | 50,000 | 20 |
Here, we choose out approximate database size and see estimated space for the AWR dba_hist tables. Oracle provides the SYSAUX sizing utility called utlsyxsz.sql (in $ORACLE_HOME/rdbms/admin) for estimating your AWR retention based on disk space consumption.
AWR also supports the creation of "AWR baselines", which are a "named" series of AWR snapshots that are immune from being purged by the AWR retention mechanism. the create_baseline procedure allows you to specify a start_snapshot and end_snapshot and name the baseline:
DBMS_WORKLOAD_REPOSITORY.create_baseline
(1900, 2000, ‘EOY_PEAK_LOADS’);
The dbms_workload_repository package can be used by Oracle DBAs to configure AWR settings such as snapshot interval, flushing and data retention.
The dbms_workload_repository .modify_snapshot_settings procedure is used to configure AWR data retention in the database. The retention interval determines the length of time that data will be preserved in the AWR. The default setting for retention is 10,080 minutes (one week).
desc dbms_workload_repository
PROCEDURE MODIFY_SNAPSHOT_SETTINGS
Argument Name Type
In/Out Default?
------------------------------
----------- ------ -------- RETENTION NUMBER IN DEFAULT
INTERVAL NUMBER IN DEFAULT DBID NUMBER IN DEFAULT
The range of valid values for this parameter ranges from ten minutes to 52,560,000 minutes or 100 years.
The dbms_workload_repository package contains the global variables of min_retention and max_retention. These variables set the lower and upper limits for the retention parameter.
If a zero value is specified for retention, snapshots will be stored for an unlimited amount of time. The following awr_settings.sql script can be used to check the current settings for the AWR interval and retention settings:
awr_settings.sql
select
extract( day
from snap_interval) *24*60+
extract( hour
from snap_interval) *60+
extract( minute
from snap_interval ) "Snapshot Interval",
extract( day from retention) *24*60+
extract( hour from retention) *60+
extract(
minute from retention ) "Retention Interval"
from
dba_hist_wr_control;
The script returns the current AWR interval values in minutes:
Snapshot Interval Retention Interval
----------------- ------------------
60 10080
Top-n SQL thresholds
The topnsql is used to specify the number of SQL to
collect at each AWR snapshot for each criteria like elapsed
time, CPU time, parse calls, shareable memory, and version
count. The topnsql is normally set to a small
number like 10, because you only want to see the most
current SQL statements. This SQL information is normally
purged after a period of time, after which the SQL source
code is no longer needed.
begin
dbms_workload_repository.modify_snapshot_settings(
retention => 7200,
interval =>60 ,
topnsql =>10 ,
dbid => 123661118);
end;
/
There are 14 criteria in an AWR report, and Oracle AWR will capture the top-n-SQL for each of these criteria:
1. Elapsed Time (ms)
2. CPU Time (ms)
3. Executions
4. Buffer Gets
5. Disk Reads
6. Parse
Calls
7. Rows
8. User I/O Wait Time (ms)
9 Cluster
Wait Time (ms)
10. Application Wait Time (ms)
11.
Concurrency Wait Time (ms)
12. Invalidations
13.
Version Count
14. Sharable Mem(KB)
You can also set threshold for each of the criteria. Unlike STATSPACK, AWR uses a "Top N" method which defaults to collect the Top-30 SQL statements for each SQL category, If you set statistics_level = "all", AWR will collect the top 100 SQL statements.
The number of SQL statements captured also depends on your statistics_level setting:
- When statistics_level=typical, AWR will capture the topnsql.
Without the topnsql set, the default is to capture 30 SQL
statements, for a total of 420 per snapshot.
- When statistics_level=all, AWR will capture the top 100
SQL for each of the criteria (elapsed time, CPU, disk reads,
etc.), for a total of 1400 SQL statements per snapshot.
Of course, some SQL statements will appear in more then one category.
http://www.dba-oracle.com/t_awr_automatic_snapshot_settings_modify.htm
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