The Complete Guide to Checksums in PostgreSQL 18 for Data Security

Data integrity is the foundation of a reliable database, and PostgreSQL has steadily strengthened its mechanisms to protect against corruption. With PostgreSQL 18, one of the most significant advancements is that checksums are enabled by default. As someone who recently compiled PostgreSQL 18 from source and tested its data integrity features using pg_checksums, I’ll walk you through a deep dive into what checksums are, how they work, and why they’re now a critical part of PostgreSQL's default setup.

What Are Checksums in PostgreSQL?

A checksum is a digital fingerprint computed for each 8KB page of data stored on disk. PostgreSQL uses a CRC32C (Castagnoli polynomial) checksum algorithm to calculate this signature, which is then stored in the page header. Every time a page is read from disk, PostgreSQL recalculates the checksum and compares it to the stored value.

If the two values don't match, it indicates corruption, either due to disk faults, memory issues, or unintended file modifications.

Corruption Types Detected by Checksums

• Silent disk corruption (common on aging or low-end hardware)
• Accidental data overwrites by rogue processes or misconfigured applications
• Bit flips due to cosmic rays or faulty RAM
• Filesystem or driver bugs

Checksums don’t fix corruption but act as an early warning system, so you can take remedial steps before data loss spreads.

Why Checksums Matter

Here’s why checksums are essential in PostgreSQL:

• Early Detection: Catch corruption during normal reads or maintenance.
• Improved Replication Safety: Standbys will reject corrupted WAL records if the checksums mismatch.
• Reliable Backups: Tools like pg_verifybackup use checksums to validate backup data.
• Recovery Validation: During crash recovery, checksums ensure WAL isn’t replaying damaged data.

By verifying page-level integrity on every read, PostgreSQL ensures you’re not unknowingly operating on bad data.

What’s New in PostgreSQL 18?

Previously, users had to opt-in for checksums during cluster initialization:

initdb --data-checksums -D /your/data/dir

You may get results like this.

bin/initdb --data-checksums -D test          
This user must also own the server process.
The database cluster will be initialized with locale "en_IN".
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".
Data page checksums are enabled.
creating directory test ... ok
creating subdirectories ... ok
selecting dynamic shared memory implementation ... posix
selecting default "max_connections" ... 100
selecting default "shared_buffers" ... 128MB
selecting default time zone ... Asia/Kolkata
creating configuration files ... ok
running bootstrap script ... ok
performing post-bootstrap initialization ... ok
syncing data to disk ... ok
initdb: warning: enabling "trust" authentication for local connections
initdb: hint: You can change this by editing pg_hba.conf or using the option -A, or --auth-local and --auth-host, the next time you run initdb.
Success. You can now start the database server using:
    bin/pg_ctl -D test -l logfile start

This meant many skipped it, especially in dev/test environments. Now in PostgreSQL 18, checksums are:

Enabled by default.

To disable them (not recommended), use:

initdb --no-data-checksums -D /your/data/dir

Upgrade Caveat

When using pg_upgrade, both source and target clusters must have the same checksum settings. Mismatches will block the upgrade. You can’t enable or disable checksums mid-upgrade.

Hands-On: Using Checksums in PostgreSQL 18 (Source Build)

Here’s how I tested checksums with my source-compiled PostgreSQL 18 setup:

1. Compile and Install PostgreSQL

./configure
make
make install

2. Initialize Cluster (Checksums Enabled by Default)

initdb -D my_data

3. Start and Use Your Cluster

pg_ctl -D my_data start

Run queries, populate data, and use your cluster normally.

4. Stop the Cluster for Verification

pg_ctl -D my_data stop

5. Check Data Integrity

pg_checksums -D my_data/

Sample Output:

Checksum operation completed
Files scanned:   950
Blocks scanned:  2892
Bad checksums:   0
Data checksum version: 1

If corruption exists, Bad checksums will be > 0, and you’ll know exactly which file or block has issues.

Key Tools and Commands for Managing Checksums

1. pg_checksums

Utility to:

• Check data file integrity
• Enable/disable checksums (offline only)

Examples:

pg_checksums --check -D demo
pg_checksums: error: data checksums are not enabled in cluster
pg_checksums --enable -D demo
Checksum operation completed
Files scanned:   950
Blocks scanned:  2884
Files written:  781
Blocks written: 2884
pg_checksums: syncing data directory
pg_checksums: updating control file
Checksums enabled in cluster
pg_checksums --check -D demo
Checksum operation completed
Files scanned:   950
Blocks scanned:  2884
Bad checksums:  0
Data checksum version: 1

2. pg_controldata

Check if checksums are enabled:

pg_controldata /your/data/dir | grep 'Data page checksum version'

1 = enabled, 0 = disabled

3. SQL Views and Functions (PostgreSQL 16+)

• pg_control_system()

Purpose:

pg_control_system() is a SQL-accessible version of pg_controldata. It exposes key metadata about the database cluster’s current control file — including:

• pg_control_version: Binary format version of pg_control.
• catalog_version_no: Version of system catalog.
• system_identifier: Unique ID for this cluster (used during replication).
• pg_control_last_modified: Last modified timestamp of the control file.

Example:

SELECT * FROM pg_control_system();

You get the result like this

pg_control_version | catalog_version_no |  system_identifier  | pg_control_last_modified  
--------------------+--------------------+---------------------+---------------------------
               1800 |          202507231 | 7533583800252464851 | 2025-08-02 09:08:59+05:30
(1 row)

Purpose:

pg_stat_io is a new system view introduced in PostgreSQL 16+ that tracks detailed I/O statistics — reads, writes, fsyncs, extends, and cache hits — broken down by:

• backend_type (e.g., autovacuum, client backend)
• object (e.g., relation, temp, wal)
• context (e.g., bulkread, bulkwrite, normal)

Relevance to Checksums:

Checksums work during read I/O. PostgreSQL will:

• Read a data page
• Compute its checksum
• Compare it with the stored checksum on disk
• Raise an error (ERROR: invalid page checksum) if there’s a mismatch

So pg_stat_io helps you observe read-heavy patterns, which may help diagnose corruption triggers

select * from pg_stat_io;

4. pg_verifybackup

Validate physical backups using checksums:

pg_verifybackup /path/to/backup

Performance Impact

Modern checksum implementation in PostgreSQL is lightweight:

• Adds ~1–2% CPU overhead during reads
• Minimal impact during writes
• Doesn’t increase disk usage (stored in page headers)

Unless you’re on very constrained hardware, the tradeoff is easily worth the safety.

In PostgreSQL 18, data checksums have moved from a best practice to a built-in safeguard, no longer optional, but essential. They form the first line of defense against silent data corruption, offering peace of mind that what’s written to disk is what will be read back. Whether you're running production systems or testing in staging environments, enabling and verifying checksums should be part of your standard database hygiene. Use tools like pg_checksums, pg_verifybackup, and robust monitoring solutions to continuously validate data integrity.

Whether you're compiling from source, using official packages, or deploying via containers, make checksums non-negotiable. In an age where data reliability is everything, trusting your database starts with protecting it.