- 28 Nov, 2025
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Fsync & Durability in PostgreSQL
Fsync, fsync=off Risks & How PostgreSQL Handles Durability
Durability is one of the most critical aspects of any relational database system. PostgreSQL ensures that once a transaction is committed, the data is safely stored—even during crashes. A key component behind this reliability is fsync, a setting that determines how PostgreSQL synchronizes data to disk.
This topic is an essential part of postgresql for dba learning paths and is deeply covered in most postgresql dba online training programs because it directly affects database reliability, performance, and crash recovery behavior.
What is Fsync in PostgreSQL?
fsync instructs PostgreSQL to physically flush data from memory to disk during every commit.
Its purpose is simple:
👉 Guarantee durability
👉 Prevent data loss during power failure or OS crash
When fsync=on (default), PostgreSQL ensures each WAL record is safely written to disk before confirming a COMMIT.
This behaves according to ACID properties, making PostgreSQL highly reliable in production.
What Happens When You Set fsync=off?
DBAs sometimes disable fsync to speed up loading operations, testing, or bulk imports.
But this comes with serious risks.
Risks of fsync=off
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You might lose committed transactions
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WAL files may become corrupt
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PostgreSQL may fail to start after a crash
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Data files may be left in an inconsistent state
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Leading to unrecoverable data loss
This is why in postgresql dba online training, instructors emphasize that fsync=off should never be used in production.
Why Some Developers Consider fsync=off?
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Faster inserts in development
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Speeding up test environments
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Improving benchmarking performance
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Bulk data loading (with caution)
Even for such scenarios, it is recommended to:
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Keep backups
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Use separate test environments
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Re-enable fsync immediately after operations
How PostgreSQL Ensures Durability with Fsync
When fsync=on, PostgreSQL uses several mechanisms to guarantee durability:
1. Write-Ahead Logging (WAL)
Changes are first written to WAL before being applied to data files.
2. WAL Buffers and WAL Writer Process
Dedicated processes flush data safely and efficiently.
3. Checkpoints
Periodic synchronization of dirty buffers to disk.
4. Reliable Sync Methods (fdatasync, fsync, open_sync)
PostgreSQL carefully chooses the best sync method based on OS and hardware.
5. Crash Recovery Using WAL Records
PostgreSQL replays WAL after a crash to restore a consistent state.
These techniques make PostgreSQL extremely resilient—even during abrupt shutdowns.
This level of durability is why learning postgresql for dba concepts is essential for anyone aiming to work on production databases.
Conclusion
Understanding fsync, fsync=off, and durability handling is crucial for PostgreSQL administrators. It directly impacts reliability and data safety. If you’re pursuing postgresql for dba knowledge or attending any postgresql dba online training, this topic forms the core of your foundational skills.
PostgreSQL’s durability model ensures that even after crashes, your database remains consistent and your data safe—provided the right settings are used.
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