How to Fix PostgreSQL Error: WAL Write Error

WAL (Write-Ahead Log) write errors occur when PostgreSQL cannot write to transaction log files, usually due to disk space or I/O issues. This is critical as it affects all write operations.

Impact

Prevents all database modifications, can cause database shutdown, and risks data loss if not addressed immediately.

Common Causes

1. pg_wal directory full
2. Disk space exhausted
3. I/O errors on disk
4. File permissions on WAL directory
5. Archive command failures

Troubleshooting and Resolution Steps

1. Check disk space:

   # Check WAL directory
   df -h /var/lib/postgresql/15/main/pg_wal
   
   # Check WAL file count
   ls -l /var/lib/postgresql/15/main/pg_wal | wc -l
   

2. Fix archive command:

   -- Check archiving status
   SELECT * FROM pg_stat_archiver;
   
   -- If archiving failing, fix or disable
   ALTER SYSTEM SET archive_mode = off;
   SELECT pg_reload_conf();
   sudo systemctl restart postgresql
   

3. Increase disk space:

   # Extend volume or free space
   # Move WAL to separate partition if needed
   

Additional Information

• WAL files critical for crash recovery
• Monitor pg_wal directory size
• Ensure archiving works if enabled
• Never delete WAL files manually while running

Frequently Asked Questions

Q: Can I delete old WAL files?
A: Never manually delete while PostgreSQL running. PostgreSQL manages recycling automatically.

Q: How much space should pg_wal have?
A: Depends on wal_keep_size and checkpoint settings. Monitor and ensure adequate free space.