PostgreSQL Background Processes

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• 13 Oct, 2025
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PostgreSQL Background Processes

Understanding PostgreSQL Background Processes: Postmaster, Background Writer, and WAL Writer Explained

1. Postmaster Process

The Postmaster is the main controller process in PostgreSQL. It is the first process that starts when the PostgreSQL server is launched and acts as the parent of all other processes.

Functions of Postmaster:

• Listens for client connection requests on TCP/IP or Unix sockets.

• Spawns a new backend process for each database connection.

• Monitors child processes and restarts them if necessary.

• Manages startup, shutdown, and crash recovery operations.

When you execute:

pg_ctl start

it launches the Postmaster, which then initializes all PostgreSQL background processes needed for database operations.

2. Background Writer Process

The Background Writer process is responsible for maintaining database performance by managing dirty buffers (modified pages) in shared memory.

Functions of Background Writer:

• Scans shared buffers periodically to find dirty pages.

• Writes modified pages to disk to free up space in shared buffers.

• Reduces disk I/O spikes by spreading write operations evenly.

• Keeps the system responsive during high workloads.

Configurable parameters:

bgwriter_delay = 200ms
bgwriter_lru_maxpages = 100

3. WAL Writer Process

The WAL Writer is one of the most crucial PostgreSQL background processes for ensuring data durability.
WAL stands for Write-Ahead Logging, a mechanism that logs all database changes before they are written to disk.

Functions of WAL Writer:

• Writes transaction logs from WAL buffers in memory to disk.

• Guarantees data safety by maintaining redo logs for recovery.

• Ensures consistent data even in case of a crash.

Configuration parameter:

wal_writer_delay = 200ms

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