Corrupted chunks and how much of the world you can save

What you are seeing

• Chunk file at [x,z] is missing block entity data in the console.
• A square of terrain regenerated as new land, cutting a build in half.
• The server crashes every time a specific player logs in or approaches an area.
• Failed to check session lock or errors mentioning region/r.x.z.mca.

Why it happens

Region files are written continuously while the server runs. A process killed mid-write leaves one half-updated — which is why the Kill button, a crash during a save, and running out of disk space are the three usual causes. Normal shutdowns finish their writes and do not do this.

Recovery, in order of preference

1. Restore from a backup taken before the corruption

   This is the only option that recovers the data intact. Everything below trades data for a world that loads.

2. Delete only the damaged region files

   The console names them: r.-1.4.mca and similar. Deleting one region regenerates that area as new terrain and leaves the rest of the world untouched. You lose a 512×512 block square, not the world.

3. Delete the player's data if it is one player crashing

   A corrupt playerdata file crashes the server when that person joins. Removing their .dat costs them their inventory and position, and costs nobody else anything.

4. Delete level.dat_old and rename level.dat_old back

   If level.dat itself is damaged, Minecraft keeps the previous copy alongside it. This recovers world settings without touching terrain.