The Database Writer (DBWR) process is a crucial background process in Oracle databases that manages the writing of data from the buffer cache to the datafiles.
DB Buffer Cache:
- The buffer cache is a portion of the System Global Area (SGA) that holds data blocks read from the datafiles.
It serves as a temporary storage for frequently accessed data, improving database performance by reducing physical I/O operations. - When data is modified, the corresponding blocks in the buffer cache are marked as "dirty."
DBWR's Role:
- The DBWR process is responsible for writing these dirty blocks from the buffer cache to the datafiles on disk.
- It's triggered by various events, such as:
- When the number of dirty buffers reaches a threshold.
- When a server process can't find a clean buffer to use.
- During checkpoints.
- When a tablespace is taken offline or read-only.
- During database shutdown.
- When the number of dirty buffers reaches a threshold.
Interaction with Datafiles:
- DBWR writes the modified data blocks from the buffer cache to the corresponding datafiles.
- It performs asynchronous I/O operations, meaning it doesn't wait for the write operation to complete before moving on to other tasks.
This improves database responsiveness. - DBWR can write multiple blocks at once (multiblock writes) to enhance efficiency.
Key Considerations:
- Asynchronous Writing: DBWR's asynchronous nature allows other database operations to continue without waiting for the write operations to complete.
- Write Optimization: DBWR optimizes write operations by grouping multiple block writes and writing to contiguous disk locations when possible.
- Data Consistency: While DBWR writes data to datafiles, the changes are also recorded in the redo log files by the Log Writer (LGWR) process. This ensures data consistency and recoverability in case of failures.
In Summary:
The DBWR process plays a vital role in managing the flow of data from the buffer cache to the datafiles.
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