WAL (Write-Ahead Logging) in PostgreSQL

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• 08 Oct, 2025
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WAL (Write-Ahead Logging) in PostgreSQL

What is Write-Ahead Logging in PostgreSQL?

Write-Ahead Logging in PostgreSQL is a technique used to guarantee data integrity.
Before PostgreSQL modifies any actual data files, it first records the intended changes in a WAL log file.

These log files are stored in the pg_wal directory and act as a journal that tracks every database modification.
If the system crashes, PostgreSQL replays these logs to recover the database to a consistent state.

It works on a simple principle:

Before making any changes to the actual data files, PostgreSQL first records those changes in a log file.

This ensures that even if the database crashes before writing data to disk, PostgreSQL can replay the WAL logs during recovery and restore consistency.

How Write-Ahead Logging in PostgreSQL Works

• Transaction Starts:
  A user executes an operation like INSERT, UPDATE, or DELETE.
• Changes Recorded in WAL:
  PostgreSQL logs these changes in sequential WAL files before modifying data blocks.
• WAL Flushed to Disk:
  The WAL file is written to disk — ensuring durability even if a crash occurs.
• Transaction Commits:
  Only after WAL is safely stored, PostgreSQL marks the transaction as committed.
• Crash Recovery:
  If the database crashes, it replays the WAL to restore data integrity.

This mechanism makes Write-Ahead Logging in PostgreSQL the guardian of your data’s safety.

Benefits of Write-Ahead Logging in PostgreSQL

WAL is central to PostgreSQL’s architecture because it ensures data safety, reliability, and performance at every stage of transaction processing. Here’s how it helps:

• Data Durability – WAL guarantees that every committed transaction is safely recorded on disk before acknowledging success, ensuring that no data is lost even if the system crashes unexpectedly.
• Crash Recovery – In the event of a crash or power failure, PostgreSQL replays the WAL files to restore the database to its last consistent state automatically — without manual intervention.
• Replication Support – WAL acts as the foundation for both physical and logical replication. Standby servers continuously read and apply WAL records to stay in sync with the primary database.
• Performance Efficiency – Instead of writing data randomly across the disk, PostgreSQL writes sequentially to WAL files, significantly improving write performance and reducing I/O overhead.
• Backup & PITR (Point-In-Time Recovery) – By archiving WAL files, DBAs can restore databases to any specific moment in time, providing a powerful safeguard against data corruption or accidental data loss.

WAL File Structure and Location

All WAL files are stored in:

/var/lib/postgresql/data/pg_wal/

Each WAL file is 16MB by default.
As PostgreSQL generates more WAL files, it either recycles them or archives them based on your configuration.

Key Configuration Parameters for WAL

In the postgresql.conf file, DBAs can tune several parameters related to Write-Ahead Logging in PostgreSQL:

wal_level = replica
archive_mode = on
archive_command = 'cp %p /backup/wal/%f'
max_wal_size = 1GB
min_wal_size = 80MB