Understanding PostgreSQL 13: Why Disk Space Isn't Freed After Killing VACUUM FULL

Discover why space doesn't immediately free up after killing a `VACUUM FULL` process in PostgreSQL 13 and learn how to reclaim that space effectively.
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Understanding PostgreSQL 13: Why Disk Space Isn't Freed After Killing VACUUM FULL

When managing databases in PostgreSQL, the VACUUM FULL process is crucial for reclaiming disk space by cleaning up dead tuples. However, users may sometimes encounter an issue where, after terminating the VACUUM FULL process, they observe that the disk space hasn't been reclaimed as expected. In this guide, we will explain this phenomenon and guide you through the steps to recover your disk space effectively.

The Problem: Disk Space Not Reclaimed

You may have recently experienced a situation similar to this:

You initiated a VACUUM FULL process in PostgreSQL.

The process continued for hours and consumed a significant amount of disk space (in this case, 150 GB).

Upon terminating this operation, you noticed that the expected free disk space (150 GB) was not returned.

Why Does This Happen?

The unexpected retention of disk space can be confusing. Here are the key reasons why the space isn't immediately reclaimed:

Logs Generated by Vacuum: During its operation, VACUUM FULL generates various logs and temporary files. When you kill this process, these logs may not get cleaned up immediately, leaving your disk space still occupied.

Data Structures Update Delay: PostgreSQL uses multiple data structures to manage how data is stored on disk. When you terminate a VACUUM FULL, it's possible that updates to these structures haven't been finalized, leading to space not being released right away.

Solution: Reclaiming Your Disk Space

Feeling frustrated with disk space management is common, but there are steps you can take to reclaim that lost space. Here’s how:

1. Clear Logs

The first step involves cleaning up any logs generated during the vacuumming process. While PostgreSQL generally handles these logs automatically, you may want to check if there are any residual logs causing the disk space retention. You can utilize commands such as pg_locks or check pg_stat_activity to get insights into ongoing processes and clean them up if needed.

2. Run a Standard VACUUM

After killing the VACUUM FULL, running a standard VACUUM command could help in reclaiming the disk space. This command will help remove any remaining dead tuples and potentially free up some additional disk space.

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3. Monitoring Disk Usage

It's critical to monitor and manage your disk usage actively. Use PostgreSQL’s built-in functions to analyze your disk usage and make necessary adjustments accordingly.

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This command provides a human-readable format of the total disk size used by the specified table.

4. Consider Your Configuration

If you find yourself frequently running into disk usage issues, consider looking into your autovacuum settings or frequency. Configuring automatic vacuuming can prevent large disk occupation by balancing data cleanup.

Conclusion

In summary, when you experience disk space not being freed after a terminated VACUUM FULL, remember that the logs and data structure updates play a key role. By taking proactive steps such as clearing logs, doing a standard VACUUM, monitoring your disk usage, and reviewing your configuration settings, you can reclaim that space efficiently.

With a better understanding of PostgreSQL’s vacuum process, managing your database's disk space will become an easier task.