Chunky

Chunky generates the world ahead of time, chunk by chunk. A new world is created as players walk into it — and that is the most demanding activity for the server. When a player enters a place where no one has been yet, the server must generate terrain, place trees, caves, and ores, and while doing all that, the game freezes briefly for all other players.

Chunky solves this problem by generating the world before players even get there. You select an area (the center is where you're standing, plus a radius) and let it work. The server keeps running in the meantime.

Typical use: after starting a new server, after adding a new world, after expanding the world border, or after a Minecraft update that changes the generator (new biomes, caves). Generating ahead of time means players won't experience freezes and the server behaves smoothly.

Chunky can work in the background with limited load (how many parts of the world per unit of time), so it can be left running even on a live server — just more slowly.

When players report falling into the void or that the landscape has holes, it is usually a sign that the area has not yet been fully generated, or that the server didn't have time for it.

Watch out for one thing: the larger the area you pregenerate, the more disk space the world takes up. For large radii, expect tens of GB.

1. Download Chunky from modrinth.com/plugin/chunky in the version for your server version (it also exists for Fabric and Forge as a mod).
2. Upload the .jar to the plugins folder and restart the server.
3. Verify that it is running: /chunky vyp prints help and status.
4. Decide what you want to generate: the whole world, or just the area around spawn. To start, the surroundings are enough — it finishes sooner.
5. Stand where the center should be (typically spawn) and set the radius: /chunky radius 500 (in blocks). For a square use /chunky square.
6. Start generation: /chunky start. Plugin ozn announces that it is working.
7. Monitor progress: /chunky progress uk shows how much is done. The server continues to work normally during this.
8. When it is done, verify it on site: teleport to the edge of the area and check that the terrain is continuous (no holes, no unfinished caves).
9. For a new server, repeat the procedure for each world separately (overworld, nether, end) — each is generated separately.
10. If you also want grass, flowers, and trees (not just terrain), use nature supplementation after generating the terrain.
11. Check free disk space — a pregenerated world is significantly larger.

Generation puts a load on both the CPU and disk. On a live server, run it with a lower load (the number of world chunks per second can be limited in the configuration), or let it run at night.
Pre-generate mainly the area around spawn and the area where players go most often. Generating the entire world to its outer limits is a waste of space and time.
The larger the area, the larger the world on disk. For large radii, expect tens of GB and that backing up the world will take longer.
When players fall into the void, the area probably isn't fully generated — check /chunky progress a pif necessary provide a smaller area.
After a Minecraft update that changes the world generator, the boundary between the old and new terrain is noticeable. Pre-generating the surroundings helps, but it won't remove the boundary.
Before you start generating, think about the world border. What you generate takes up space even outside it — and inaccessible parts are then cleaned up with the command /chunky trim.
Zback up the world after generation is complete. A pre-generated world is the best state to restore from.