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Lesson 12 Limiting the number of dirty buffers
Objective How to limit the number of dirty buffers

Limiting the Number of Dirty Buffers

To effectively limit the number of dirty buffers in the database buffer cache when monitoring Oracle processes, it is essential to understand and utilize Oracle's built-in mechanisms and parameters that control buffer cache behavior. Dirty buffers are pages in the buffer cache that have been modified but not yet written to disk. Managing these effectively is crucial for maintaining database performance and ensuring data consistency. Here are the key strategies and parameters you can adjust:
  1. DBWR (Database Writer) Processes Configuration: Oracle's Database Writer (DBWR) processes are responsible for writing dirty buffers from the cache to the disk. You can configure the behavior and efficiency of these processes to ensure timely writing of dirty buffers. This can involve adjusting the number of DBWR processes (`DB_WRITER_PROCESSES` parameter) to suit the workload and hardware capabilities of your system.
  2. Checkpoint Tuning: Checkpoints are events that trigger the writing of all dirty buffers to disk. By tuning checkpoint-related parameters, you can control how frequently these events occur, thereby indirectly managing the number of dirty buffers. Key parameters include:
    • `LOG_CHECKPOINT_TIMEOUT`: This parameter specifies the maximum time interval between checkpoints. Reducing this value can increase checkpoint frequency, reducing the number of dirty buffers.
    • `LOG_CHECKPOINT_INTERVAL`: This parameter determines the number of redo log file blocks that can be filled before a checkpoint is triggered. Adjusting this value can help manage the checkpoint frequency and the accumulation of dirty buffers.
  3. Redo Log Configuration: The size and number of redo log files can impact checkpoint behavior and thus the number of dirty buffers. Optimizing the redo log configuration to match your workload can help in ensuring more efficient checkpoints and buffer management.
  4. Automatic Memory Management (AMM) and Automatic Shared Memory Management (ASMM): Oracle provides AMM and ASMM features to manage memory allocation dynamically, including the buffer cache size. Ensuring these features are properly configured allows Oracle to adjust memory allocations based on workload patterns, which can indirectly affect the number of dirty buffers by optimizing buffer cache usage.
  5. Using the `DB_CACHE_ADVICE` Utility: This Oracle utility provides statistics on the usage and effectiveness of the buffer cache at different sizes. By analyzing this data, you can make informed decisions about adjusting the buffer cache size to optimize the balance between hit ratios and the management of dirty buffers.
  6. Monitoring and Diagnostics Tools: Regularly use Oracle's performance monitoring and diagnostic tools, such as Automatic Workload Repository (AWR) reports and the Active Session History (ASH), to monitor the behavior of dirty buffers and the efficiency of DBWR processes. This data can guide you in fine-tuning the system parameters for optimal performance.
  7. I/O Subsystem Tuning: Ensure that the underlying I/O subsystem is optimized for high performance. Efficient disk storage setups, such as using RAID configurations and solid-state drives (SSDs), can significantly reduce the I/O latency for writing dirty buffers to disk, thus allowing more aggressive checkpointing and buffer management strategies without impacting overall performance.

By carefully managing these aspects of your Oracle database environment, you can effectively limit the number of dirty buffers in the database buffer cache, ensuring smooth operation and optimal performance of your Oracle processes.

You can use the DB_BLOCK_MAX_DIRTY_TARGET parameter to place an upper limit on the number of dirty buffers in the database buffer cache. You set this parameter in your initialization file, like this:
The example above limits the maximum number of dirty blocks to 200. The default behavior is to have no upper limit at all.
In this context, blocks and buffers are interchangeable. Strictly speaking, a buffer is an area in memory, a block is a piece of data from disk. However, Oracle always sizes the buffers in the buffer cache to match the block size. 200 dirty blocks is equal to 200 dirty buffers.

Why limit dirty buffers?

The number of dirty buffers has a direct impact on the amount of time needed for crash recovery.
That is because when the server crashes, it's the buffered data that you lose. When you restart the instance, those lost changes need to be reconstructed based on information in the redo logs. The more dirty buffers that are lost, the longer recovery will take.

Recovery time versus performance

If you are in a time-sensitive environment, you might use the DB_BLOCK_MAX_DIRTY_TARGET parameter to keep the number of dirty blocks, and thus the recovery time, low. Of course this comes at a cost. Performance might suffer somewhat during periods of high activity, because Oracle will occasionally need to wait for the database writer to lower the dirty block count (by writing buffers back to the datafiles).


Property Description
Parameter typeInteger
Default value1 or CPU_COUNT / 8, whichever is greater
Modifiable No
Range of values 1 to 20
Basic No

DB_WRITER_PROCESSES is useful for systems that modify data heavily. It specifies the initial number of database writer processes for an instance.

DBwriter Process -Quiz

Click the Quiz link below to test what you have learned about the DBWR and database buffers.
DBwriter Process - Quiz

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