This event shows a wait for a foreground process while doing a sequential read from the database. The I/O is generally issued as a single I/O request to the OS; the wait blocks until the I/O request completes.
The rest of the information in this section is only valid for this metric when it appears in either the Enterprise Manager Grid Control or the Enterprise Manager Database Control (if applicable).
The following table shows how often the metric's value is collected and compared against the default thresholds. The 'Consecutive Number of Occurrences Preceding Notification' column indicates the consecutive number of times the comparison against thresholds should hold TRUE before an alert is generated.
|
Target Version |
Evaluation and Collection Frequency |
Upload Frequency |
Operator |
Default Warning Threshold |
Default Critical Threshold |
Consecutive Number of Occurrences Preceding Notification |
Alert Text |
|
pre-10g |
Every Minute |
After Every Sample |
> |
Not Defined |
Not Defined |
3 |
%value%%% of service time is spent waiting on the 'db file sequential read' event. |
(DeltaDbFileSequentialReadTime/DeltaServiceTime)*100 where:
DeltaDbFileSequentialReadTime: difference of 'sum of time waited for sessions of foreground processes on the 'db file sequential read' event' between sample end and start
DeltaServiceTime: difference of 'sum of time waited for sessions of foreground processes on events not in IdleEvents + sum of 'CPU used when call started' for sessions of foreground processes' between sample end and start
See Idle Events
Because I/O is a normal activity, take notice of unnecessary or slow I/O activity. If the TIME spent waiting for I/Os is significant, then it can be determined for which segments Oracle has to go to disk. See the "Tablespace I/O" and "File I/O" sections of the ESTAT or STATSPACK reports to get information on which tablespaces and files are servicing the most I/O requests, and to get an indication of the speed of the I/O subsystem.
If the TIME spent waiting for reads is significant, then determine against which segments Oracle is performing the reads. The files where the reads are occurring can be found by looking at the V$FILESTAT view.
Also, see which sessions are performing reads and trace them to see if the I/Os are expected. You can use this statement to see which sessions are worth tracing:
SELECT sid, total_waits, time_waited FROM v$session_event WHERE event='db file sequential read' and total_waits>0 ORDER BY 3,2 ;
You can also look at:
Statements with high DISK_READS in the V$SQL view
Sessions with high "physical reads" in the V$SESSTAT view
Related Topics
About Alerts
About the Metric Detail Page
Editing Thresholds
Understanding Line Charts
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