Chunk corruption (ChunkSerializationException / DFU migration failed)
Chunk corruption happens when Minecraft can't read or write a region file — usually because a disk-full crash, a power cut, or a save-interrupted-by-kill left a partial .mca file. The server either refuses to start (if the corrupt chunk is in the spawn region) or crashes when a player loads it. The corrupt region file has to be either repaired or deleted, regenerating the terrain inside it.
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How to fix it
- 1
Identify the corrupt region file from the crash
The error logs the chunk coordinates. Convert them to a region file: region X = floor(chunkX/32), region Z = floor(chunkZ/32), file = world/region/r.X.Z.mca. For nether/end chunks, the path is world/DIM-1/region/ or world/DIM1/region/.
- 2
Check if the region file is actually truncated
Run 'ls -lh world/region/r.X.Z.mca'. A healthy region file is at least a few KB. A file of 0 bytes or a few hundred bytes was truncated mid-write. That file is the problem.
- 3
Restore from backup if you have one
The safest fix is restoring just the affected .mca file from the last good backup. If your host keeps automated backups (CoalHosting does, configurable in Settings → Backups), download the backup, extract the specific .mca file, and replace the corrupt one. Players lose progress only in that region since the last backup.
- 4
Delete the corrupt region file to regenerate terrain
If no backup is available, stop the server and delete the corrupt .mca file. Minecraft regenerates the terrain in that region on next boot. Players lose builds and loot in that region, but the rest of the world is untouched. This is the nuclear option — only do this if restoration failed.
Alternative causes
These can produce the same error message — worth ruling out if the steps above don't resolve it.
DFU (DataFixerUpper) migration crash
When loading worlds from older MC versions, DFU converts old data formats. If a chunk's NBT is too malformed for DFU to parse, it throws a migration error. The fix is the same (delete or restore the region file), but also check whether a mod format-conversion was expected for this world.
Disk full during a save
If the disk ran out of space exactly while writing a region file, the file ends mid-record. Fix the disk-space issue first (see our disk-full guide), then delete or restore the corrupt file.
Frequently asked
How many chunks are in one region file?
Each .mca file contains a 32×32 grid of chunks, so 1,024 chunks. Deleting one region file regenerates at most a 512×512 block area.
Will other players' builds in the same region be lost?
Yes — the region file holds all 1,024 chunks in that grid. If players built anything in those 512×512 blocks, it regenerates to default terrain. Check coordinates against known base locations before deleting.
Can I repair a region file without deleting it?
Tools like Amulet Editor and Chunker can scan a region file and zero out or replace only the malformed chunks, preserving the valid ones. This takes more effort but saves builds in the non-corrupt chunks of the same region.
Want this auto-fixed on your server?
CoalHosting's Minecraft hosting runs the same pattern database against every crash and applies the known fix before your players notice. Free crash diagnosis works on any server, hosted or not.
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Last reviewed 2026-06-15. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.