This event occurs mainly when a server process is trying to read a new buffer into the buffer cache but too many buffers are either pinned or dirty and thus unavailable for reuse. The session posts to DBWR then waits for DBWR to create free buffers by writing out dirty buffers to disk.
DBWR may not be keeping up with writing dirty buffers in the following situations:
The I/O system is slow.
There are resources it is waiting for, such as latches.
The buffer cache is so small that DBWR spends most of it's time cleaning out buffers for server processes.
The buffer cache is so big that one DBWR process is not enough to free enough buffers in the cache to satisfy requests.
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 |
> |
20 |
Not Defined |
3 |
%value%%% of service time is spent waiting on the 'free buffer waits' event. |
(DeltaFreeBufferWaitsTime/DeltaServiceTime)*100 where:
DeltaFreeBufferWaitsTime: difference of 'sum of time waited for sessions of foreground processes on the 'free buffer waits' 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
Sometimes the easy solution is to increase the buffer cache to allow for more free blocks. This works in many cases, but if the application is generating a sustained amount of dirty blocks then increasing the buffer cache may only help or delay the problem but not solve it.
If this event occurs frequently, examine the session waits for DBWR to see whether there is anything delaying DBWR.
Run this query to see if the I/O is evenly distributed.
SELECT name, phyrds, phywrts FROM v$filestat a, v$datafile b WHERE a.file# = b.file#
Also look for files having full table scans, using this query:
SELECT name, phyrds, phyblkrd, phywrts FROM v$filestat a, v$datafile b WHERE a.file# = b.file# AND phyrds != phyblkrd
Related Topics
About Alerts
About the Metric Detail Page
Editing Thresholds
Understanding Line Charts
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