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Lesson 8 Database Writer (DBW0)
Objective Identify the purpose of the DBW0 process.

(DBW0) Database Writer

Lesson 8: Oracle Database Writer (DBWn)

Objective

Identify the purpose and functionality of the Database Writer (DBWn) process in Oracle databases.

Overview of the Database Writer (DBWn)

The Database Writer (DBWn) process is a critical background process in Oracle databases, responsible for writing modified (dirty) data blocks from the database buffer cache in the System Global Area (SGA) to the datafiles on disk. This ensures data consistency, durability, and compliance with Oracle’s ACID (Atomicity, Consistency, Isolation, Durability) principles, while freeing up buffer cache space for new operations.

Key Features

How DBWn Works

The DBWn process periodically scans the buffer cache for dirty buffers—data blocks modified by operations like UPDATE, INSERT, or DELETE—and writes them to the datafiles. This process is illustrated in the following sequence:

Empty database buffers at startup
Figure 1: When a database starts, all buffers in the cache are empty.
Cache fills with data blocks
Figure 2: As queries execute, data blocks are read from disk into the buffer cache.
Data blocks modified
Figure 3: UPDATE, INSERT, and DELETE statements modify data blocks, marking them as dirty.
DBWn writes dirty blocks to disk
Figure 4: DBWn periodically writes dirty buffers to disk.
Modified blocks retained in cache
Figure 5: Written blocks remain in the cache for potential reuse.
DBWn catches up with changes
Figure 6: When activity slows, DBWn ensures all changes are written, keeping the cache up to date.

Configuring Multiple Database Writers

For databases with high update activity or large buffer caches, Oracle allows multiple Database Writer processes or I/O slaves to distribute the workload efficiently.

Multiple DBWn Processes

Oracle supports up to 20 Database Writer processes (DBW0 to DBW9 and DBWa to DBWj) to handle high I/O demands, particularly on multi-CPU systems. The number of processes is set using the DB_WRITER_PROCESSES initialization parameter.

DB_WRITER_PROCESSES=5
    

This configures five writer processes: DBW0, DBW1, DBW2, DBW3, and DBW4. Multiple writers are effective on systems with multiple CPUs, as they distribute I/O tasks across processors. Oracle recommends at least one DBWn process per 8 CPUs or per processor group.

I/O Slaves

On single-CPU systems or when asynchronous I/O is unavailable, Oracle can use I/O slaves to offload I/O tasks from the DBWn process. The DBWR_IO_SLAVES parameter controls the number of slaves:

Note: Asynchronous I/O at the operating system level is preferred when available, as it reduces the need for I/O slaves.

When to Use Multiple Writers or Slaves

Performance Considerations

The efficiency of the DBWn process directly impacts database performance:

Key Takeaways

Notes:

  • For detailed configuration guidance, refer to Oracle’s official documentation at https://www.oracle.com/database/.
  • Monitor DBWn performance using views like V$SYSSTAT or V$SESSION_EVENT to identify I/O bottlenecks.

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