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Minecraft server FAQ

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Contents

General

How do I start a Bukkit server?

See the corresponding tutorial.

What are these "Can't keep up" messages?

These messages indicate instances of when your Minecraft server detects that the server clock has fallen behind. Receiving these infrequently is not a sign of worry, but if you receive them very often and consistently, then it may be a sign of major problems. If the clock is sufficiently behind, then you may have problems such as blocks respawning when you're mining and a slow day/night cycle.

Tip: To accurately measure your server's clock rate, Bukkit users can use CommandBook's /debug clock command to give an measurement.

Three potential causes of a slow-running clock are:

  1. Your system may not be capable of running the Minecraft server. You generally need several gigabytes of RAM and a CPU with a fast clock speed for smooth performance. Be aware that Minecraft is not multi-threaded, so a CPU with 8 cores will not do you any good. However, a 3 GHz CPU will do better than a 2.5 GHz CPU.
  2. There may be something in the world that is eating up resources in the server, such as an excessive number of mobs. Try running your server with a fresh world to see if the problem disappears. If you suspect that the trouble lies with your world, check out "What are the causes of lag?".
  3. You have a poorly-written modification or plugin that is consuming excessive resources. Try removing plugins and modifications to see if it solves your problem. If it does, then restore modifications until you find the troublesome one.

Why do blocks respawn when I mine them?

There are generally three causes, listed in order that you should check for:

  • First, check to see if spawn protection is enabled, which protects blocks near the spawn point. If you are using the vanilla Minecraft server, then it's not possible to disable spawn protection, but if you are using a server modification (such as hMod or Bukkit), then it is possible to disable this feature. Only ops can build and mine in the spawn protected area.
  • Your server's clock could be running slow. See the question above for more information, including a method to measure your internal MC clock rate.
  • You have a server modification or plugin that is denying the mining attempt. Remove all modifications and plugins that you have added to see if it solves your problem. If it does, then re-add modifications and plugins until you identify the offending one.

Why can't I place or mine blocks?

Make sure you are not near the spawn point and haven't yet turned off the spawn protection (which is on by default). If that isn't your problem, disable modifications and plugins to see if any of them cause the problem.

If you are able to place blocks but not mine them reliably, them check out "Why do blocks respawn when I mine them?".

How can I tell how well my server is running?

Minecraft has an internal clock that runs at a rate of 20 cycles per second. A server that is running well will generally run around that number, dipping perhaps to 19.7 ticks per second. To accurately measure your server's clock rate, Bukkit users can use CommandBook's /debug clock command.

Why do I overflow when I approach an area? (disconnect.overflow)

You likely have too much flowing water or lava. See "What are the causes of lag?" and look for the entry about flowing water and lava.

How do I get rid of a ton of dropped items?

Bukkit users can use WorldEdit near the area to remove entities (/remove drops -1, /remove arrows -1, etc.). If you are able to shut off your server, you can use mce.py to remove all entities, although this will also remove paintings.

What are these "GzipException" messages when I start my server?

Minecraft stores the world in 16 by 16 by 128 block regions called chunks. These messages indicate that some of these chunks are corrupt, a problem that usually occurs because the Minecraft server was not properly shut down. Worlds in the Alpha and Beta (pre-1.3) format can use checkworld.py. However, starting in 1.3, chunk files are now bundled into one large "McRegion" file, for which no equivalent checkworld.py tool exists yet.

How can I reduce bandwidth usage?

Disabling mobs should reduce bandwidth usage significantly.

What is the cause of java.net.SocketException: Connection reset?

This "error" only means that a player disconnected. The technical reason for this error is that the player closed Minecraft but Minecraft did not properly tell the server that it had disconnected. The server gets told to abruptly "reset" (and shut off) the connection when it tries to send a message to the player. Most server software ignore this error (since it's unimportant), but the Minecraft server spits it out.

What are the causes of lag?

Cause Description Prognosis Type of lag
Poorly written server plugin or modification Problematic addons could cause a number of problems. Remove modifications and plugins to see if they cause the problem. Remove all at first to rule out this cause. High server CPU usage, extra network traffic
Excessive number of mobs (monsters and animals) Mobs and animals can often constitute hundreds of entities, causing high CPU usage and extra network usage. Squids in particularly are often numerous and also consequently expensive. Try disabling monster and animal spawning to see if it makes a difference. Bukkit users can use WorldEdit near the area to remove mobs in loaded chunks (/butcher). If you are able to shut off your server, you can use mce.py to remove all entities, although this will also remove paintings. High server CPU usage, extra network traffic
Excessive number of item drops, arrows, or other entity The client cannot handle many entities very well and the server won't take it better. A failed operation (such as in WorldEdit) could potentially drop a large number of items (such as cacti items if cacti is not placed in a valid location). Having too much fun with arrows can leave many arrows around. If you are near the area (and experience client lag), you can press F3 and check the denominator of "E" on the third line to see the number of entities in the area. If it is over 1000, you may have this problem. Use mce.py to remove all entities, although this will also remove paintings. Bukkit users can use WorldEdit near the area to remove entities (/remove drops -1, /remove arrows -1, etc.) High server CPU usage, severe client framerate drop
Excessive flowing water and lava Water and lava calculation is expensive due to sheer amount of it. However, if there is an excess of flowing water or lava in one area, it will also causes clients to overflow due to the amount of data that has to be sent . A common symptom of this problem is to be overflowed when you approach an area that has the problem (the message in console will be "disconnect.overflow"). You can look at the graph at the bottom of F3 to see if there is an abundance of tall red lines, potentially confirming this problem. Bukkit users can also attempt to use WorldEdit near the area to remove water by using /removenear water 90 and /removenear movingwater 90 repeatedly (as you get nearer). This will result in huge swaths of waterless areas, however, and it may be difficult to do. High server CPU usage, connection overflows, client framerate drop
Excessive number of cacti placed next to each other Illegally placed cacti next to each other will cause very severe server lag. Cacti cannot normally be placed in the game to this extent, but an external tool or a modification may allow it. Look for large swaths of cacti and remove as necessary. High server CPU usage, severe client framerate drop
Excessive redstone usage Using an excessive amount of redstone (namely redstone circuits that use a clock or pulser) will cause a lot of network traffic and could potentially be a cause of high server CPU usage. Look for clocks, pulsers, and other redstone circuits that are self-powered. Greater server CPU usage, severe network traffic, framerate drop
Excessive falling sand and gravel If sand or gravel is falling, it will cause network traffic in order to update nearby clients Normally sand and gravel are all settled so this isn't usually a problem, but sometimes there are large formations of floating and or gravel. A player may also be using a tool to create larges amount of sand and gravel at a time. Try to find falling sand and gravel, but this problem, if it exists, is not very prevalent. Severe network traffic
Chunk generation Generating new chunks consumes a lot of CPU. Walking towards the end of the existing generated world will cause new chunks to generate. Upgrade your system or restrict exploring. High server CPU usage, framerate drop
Raging fire A raging fire sends a lot of network data and may consume a lot of CPU due to fire's nature of spreading rapidly. Use WorldEdit's /ex command near the area to remove the fire. Alternatively, you can shut down the server and use mce.py or MCedit to remove the fire. Extra CPU usage, Connection overflows, high network traffic
Igniting TNT TNT can cause a lot of lag, both on the client and on the server. Look for ongoing TNT explosions in the world in order to remove them. Extra CPU usage, high network traffic, severe client frame rate drop
Lighting changes Lighting changes cause block data to be sent, causing heavy CPU usage. Look for redstone circuits that are hooked up to torches, or if CraftBook is installed, pumpkins. Eliminate any that you see that you suspect is causing the problem. High network traffic, client frame rate drop

Emergency response

How do I "unfreeze" my server after TNT was ignited?

You should kill your server in order to stop the TNT. Afterwards, try booting it up again to see if the TNT problem has disappeared. If not, use mce.py to remove entities (note: this will remove paintings as well).

A fire has started raging; what do I do?

WorldGuard users can use the /stopfire command to cease all fire spread. Use WorldEdit's /ex command near the area to remove the fire. Alternatively, you can shut down the server and use mce.py or MCedit to remove the fire.



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