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world/session.lock already locked

Minecraft takes an exclusive lock on the world through session.lock. When the server starts and finds the lock held, it refuses rather than risk two processes writing the same world. The lock is usually stale — left by a crashed instance — but treating it as stale without checking is how worlds get corrupted.

Fastest path: paste your crash report into the Crash Doctor — it identifies this pattern and 40+ others automatically. No signup.

How to fix it

  1. 1

    Verify no instance is actually running

    Check the panel status and, on self-hosted boxes, the process list for stray java processes. A second instance sharing the folder is the dangerous case.

  2. 2

    Wait 60 seconds and Start

    On CoalHosting the health system auto-cleans stale locks on the next sweep; a short wait plus Start resolves the ordinary case.

  3. 3

    Stop any backup or file-sync tool

    An off-site backup walking the world folder can hold the lock mid-run. Pause it, then Start the server; resume the backup afterwards with the server stopped or snapshot-safe.

  4. 4

    If it keeps recurring

    A lock that returns every boot means something on the machine starts a second process — a watchdog script, a wrapper, a second panel agent. Find and disable it before it writes.

Alternative causes

These can produce the same error message — worth ruling out if the steps above don't resolve it.

Panel started the server while a manual run was alive

Starting from the panel while you were testing java -jar manually over SSH collides exactly here. One owner of the world at a time.

Shared-folder or NFS hosting quirks

Locks on network shares are unreliable. If the server directory lives on NFS, move it to local disk — both locking and performance improve.

Frequently asked

Is my world corrupted?

Almost certainly not. The lock refused a second writer, which is exactly what protects the world. Corruption fears are only warranted if two processes wrote simultaneously.

What is session.lock?

A tiny file whose content timestamps the world's active session. Minecraft uses it as a cooperative flag: only one process may hold it.

Can I just delete session.lock?

Only with the server fully stopped and no other process touching the folder. Deleting it while a process is writing is one of the fastest ways to lose a world.

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 2026-09-06. If a step is wrong or out of date, tell us — we'll fix the article and the auto-pattern at the same time.