Oracle wait class tips Oracle Tips by Burleson Consulting

Not All Events Are Created Equal

The information contained in the AWR is substantial and over 800 distinct wait events are tracked.  To facilitate the use of these events, they have been grouped into 12 areas called Wait Classes.  These classes are listed in Table 16.2.

More detailed ASH information on waits that are occurring in a specific wait class is available in other areas of the repository. The following script lists the specific wait events that are part of each wait class. 

With over 800 events, this could be a bit overwhelming, so it may be advisable to filter this query with a WHERE clause to restrict the output to the wait class that of the most interest to the DBA.  Some of the events for the System I/O and User I/O are shown below as an example:

WAIT_CLASS    EVENT_NAME                              

----------    ------------------------------

System I/O    db file parallel write                  

              io done

              kfk: async disk IO

              ksfd: async disk IO

              log file parallel write

              log file sequential read

              log file single write

              recovery read

User I/O      BFILE read     

              buffer read retry

              db file parallel read

              db file scattered read

              db file sequential read

              db file single write

The listing above is useful if the AWR top five timed events report indicates a significant amount of time spent on I/O related waits. 

Here is another service wait class script to display waits by day using the AWR dba_hist_service_wait_class table:

col c1 heading 'end|time'    format a10
col c2 heading 'wait|class'  format a20
col c3 heading 'time|waited' format 999,999,999,999

break on c1 skip 2

select
   trunc(end_interval_time)  c1,
   wait_class                c2,
   sum(time_waited)          c3
from
   dba_hist_service_wait_class
join
   dba_hist_snapshot USING(snap_id)
group by
   trunc(end_interval_time),
   wait_class
order by
   trunc(end_interval_time),
c3 desc;

Here is a sample of the output.  This shows the "landscape" of what is happening on the database:

end        wait                 time
time       class                waited
---------- -------------------- ----------------
31-OCT-12  Idle                  2,569,604,134
           System I/O               21,125,983
           Other                       134,514
           User I/O                    126,006
           Concurrency                  98,622
           Commit                       24,102
           Scheduler                     3,458
           Configuration                 1,273
           Application                     140
           Network                         126


01-NOV-12  Idle                  3,202,677,185
           System I/O               22,725,490
           Other                       138,425
           User I/O                    119,588
           Concurrency                 115,050
           Commit                       28,105
           Configuration                 3,991
           Scheduler                     3,211
           Application                     130
           Network                         117

One also has to filter out wait events that are not helpful to the tuning effort.  In practice, idle events can be filtered out by adding where wait_class <> ‘Idle’ in the ASH queries.  Table 16.3 below shows all system idle wait events that have usually no meaningful information in Oracle bottleneck analysis.  One exception might be a batch program where idle events may indicates that the program is doing something large which makes RDBMS wait. Remember, SQL*Net message to client waits almost always indicates network contention.

The next section will provide a look inside the most useful ASH tables for time-series wait event tuning.

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