This metric represents the number of times, per second, during this sample period DBWn was asked to scan the cache and write all blocks marked for a checkpoint.
The database writer process (DBWn) writes the contents of buffers to datafiles. The DBWn processes are responsible for writing modified (dirty) buffers in the database buffer cache to disk.
When a buffer in the database buffer cache is modified, it is marked dirty. The primary job of the DBWn process is to keep the buffer cache clean by writing dirty buffers to disk. As user processes dirty buffers, the number of free buffers diminishes. If the number of free buffers drops too low, user processes that must read blocks from disk into the cache are not able to find free buffers. DBWn manages the buffer cache so that user processes can always find free buffers.
When the Oracle Server process cannot find a clean reusable buffer after scanning a threshold of buffers, it signals DBWn to write. When this request to make free buffers is received, DBWn writes the least recently used (LRU) buffers to disk. By writing the least recently used dirty buffers to disk, DBWn improves the performance of finding free buffers while keeping recently used buffers resident in memory. For example, blocks that are part of frequently accessed small tables or indexes are kept in the cache so that they do not need to be read in again from disk. The LRU algorithm keeps more frequently accessed blocks in the buffer cache so that when a buffer is written to disk, it is unlikely to contain data that may be useful soon.
Additionally, DBWn periodically writes buffers to advance the checkpoint that is the position in the redo log from which crash or instance recovery would need to begin.
This test checks the number of times DBWR was asked to advance the checkpoint. If the value is greater than or equal to the threshold values specified by the threshold arguments, and the number of occurrences exceeds the value specified in the "Number of Occurrences" parameter, then a warning or critical alert is generated.
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 5 Minutes |
After Every Sample |
> |
Not Defined |
Not Defined |
2 |
DBWR checkpoint rate is %value%/sec. |
|
Target Version |
Server Evaluation Frequency |
Collection Frequency |
Upload Frequency |
Operator |
Default Warning Threshold |
Default Critical Threshold |
Consecutive Number of Occurrences Preceding Notification |
Alert Text |
|
10.1.0.x; 10.2.0.x; 11.1.0.x; 11.2.0.x |
Every Minute |
Not Defined |
After Every Sample |
> |
Not Defined |
Not Defined |
2 |
Generated By Database Server |
DeltaCheckpoints / Seconds where:
DeltaCheckpoints: difference in 'select value from v$sysstat where name='DBWR checkpoints'' between sample end and start
Seconds: number of seconds in sample period
A checkpoint tells the DBWR to write out modified buffers to disk. This write operation is different from the make free request in that the modified buffers are not marked as free by the DBWR process. Dirty buffers may also be written to disk at this time and freed.
The write size is dictated by the _db_block_checkpoint_batch parameter. If writing, and subsequently waiting for checkpoints to complete is a problem, the checkpoint completed event displays in the Top Waits page sorted by Time Waited or the Sessions Waiting for this Event page.
If the database is often waiting for checkpoints to complete you may want to increase the time between checkpoints by checking the init.ora parameter db_block_checkpoint_batch: select name, value, is default from v$parameter where name = db_block_checkpoint_batch. The value should be large enough to take advantage of parallel writes. The DBWR uses a write batch that is calculated like this: (db_files * db_file_simultaneous_writes)/2 The write_batch is also limited by two other factors:
A port specific limit on the numbers of I/Os (compile time constant).
1/4 of the number of buffers in the SGA.
The db_block_checkpoint_batch is always smaller or equal to the _db_block_write_batch. You can also consider enabling the check point process.
Related Topics
About Alerts
About the Metric Detail Page
Editing Thresholds
Understanding Line Charts
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