DataFixerUpper and long world loading
What the data converter does when changing versions, why startup takes minutes, and when this is a sign of a problem.
Updated August 13, 2026
[INFO]: Upgrading data for level.dat
[INFO]: Converting chunks: 15%
DataFixerUpper is Mojang's mechanism that brings data from an older version into the format of a newer one. It runs automatically the first time a world starts on the new version.
This is normal when
- You have just changed the game version — Updating the server.
- The world is large: converting tens of thousands of chunks takes minutes, sometimes tens of minutes.
- The process runs with visible progress and finishes.
This is a problem when
| Symptom | Cause |
|---|---|
| Conversion starts on every startup | The world isn't being saved: check write permissions and free disk space |
| The process hangs at one percent | Corrupted chunk — Region file corrupted |
| Errors during the process | The world is partially corrupted — Corrupted world |
| Memory runs out during conversion | You need a larger plan for the duration of the conversion — OutOfMemoryError |
What to do in advance
- A copy of the world before changing the version is mandatory.
- Run the conversion at a quiet time: while it runs, the server is unavailable.
- On a modpack, conversion also means recalculating registries — Registry remapping.
Speeding it up
There's no way. The only thing you can do is avoid running it on a live server during prime time.
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