🧠 4.PostgreSQL summarize_wal — Overview

summarize_wal is a key component enabling incremental backups in PostgreSQL (v17+). It allows backups to capture only the blocks that have changed since the last backup, improving speed and efficiency.

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🔍 How It Works

• The summarize_wal process scans WAL (Write-Ahead Log) files.
• It reads changes from the last REDO point to the current REDO point.
• It generates summary files that describe which data blocks were modified.
• These summary files are stored in:
• $PGDATA/pg_wal/summaries/

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⚙️ Controlled By

• **summarized_wal** configuration parameters in postgresql.conf.

• Settings include when and how WAL summarization runs (e.g., frequency, retention, etc.).Demo :

  ✅ Step-by-Step: Enabling and Using summarize_wal

  1. Check Current Setting

  SHOW summarize_wal;
  This shows whether WAL summarization is currently enabled (on) or disabled (off).

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  2. Enable WAL Summarization

  ALTER SYSTEM SET summarize_wal = 'on';
  This command enables WAL summarization system-wide.

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  3. Reload PostgreSQL Configuration

  SELECT pg_reload_conf();
  Applies the change without restarting the server.

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  4. Verify the Change

  SHOW summarize_wal;
  Should return on now.

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  5. Locate WAL Summary Files

  • After enabling this feature, PostgreSQL will create summary files in the pg_wal/summaries/ directory.
  • These files summarize activity in WAL segments and can speed up recovery, replication, or point-in-time restore operations.

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  6. Query Available Summaries

  SELECT * FROM pg_available_wal_summaries() ORDER BY start_lsn;
  Returns a list of all available WAL summaries, including:
  start_lsn and end_lsn: WAL log sequence numbers
  start_time and end_time: Time range covered
  Potentially other metadata about each summary file

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  ✅ Confirming It’s Working

  • WAL summary process should now be visible in PostgreSQL background processes.
  • You should see new files in the pg_wal/summaries/ directory, usually with .summary extensions.

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  📌 Notes

  • This is not a standard PostgreSQL 14 or lower feature.
  • Might be available in PostgreSQL 16+ or custom builds (e.g., Postgres Pro or similar).
  • Be sure your PostgreSQL version supports summarize_wal by checking the documentation or:
  • SELECT version();

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✅ Purpose

• Enables incremental backups by identifying changed blocks.

• Reduces backup size and time for large databases.

✅ Step-by-Step: Enable Incremental Backups with WAL Summaries

1. 🛠️ Set WAL Summarization Parameters

Edit your postgresql.conf file:

summarize_wal = on
wal_summary_retention = 7d          # Keep summaries for 7 days (adjust as needed)
Optionally restart PostgreSQL:

sudo systemctl restart postgresql-17

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2. 👤 Ensure You Have a Base Backup (Initial Full Backup)

This is required before any incremental backups can be taken.

pg_basebackup -U bkpadmin -D /var/lib/pgsql/17/base_backup -Ft -Xs -P --write-recovery-conf

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3. 📦 Take an Incremental Backup

Once WAL summarization is running and a full base backup exists, you can use the --incremental option:

pg_basebackup -U bkpadmin -D /var/lib/pgsql/17/inc_backup_20250730 --incremental=/var/lib/pgsql/17/base_backup/backup_manifest \
  -Ft -z -Xs -P

🔑 The --incremental flag points to the manifest file from your previous base or incremental backup.

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• 📁 What Happens Internally
• PostgreSQL reads WAL summaries in pg_wal/summaries/
• Identifies only blocks that changed since the last backup
• Produces a new tarball with just the changed blocks
• Saves a new manifest for future incremental runs

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📌 Summary

Feature	Purpose
summarize_wal = on	Enables generation of WAL summaries
pg_wal/summaries/	Stores summary files
--incremental=...	Triggers an incremental backup
pg_basebackup (PostgreSQL 17+)	Supports incremental backups natively