This post is about hosting a Minecraft server on the RasPi and not playing Minecraft client on it! There are several threads in this forum which already got into that matter. Those about the Minecraft server have some misleading topic and therefor got partially hijacked as well.
So here is my summary and progress on this topic. Of course i am standing on this forum's shoulders and other sources, which i hope to credit in the necessary places.
Minecraft Server on the RasPi
The rasPi can run an alternative Minecraft server for a small group of players. It can not run the official Minecraft server (yet). Only an vanilla game is possible using the alternative MC-Server
(no RedPower 2, Buildcraft, etc.).
After playing around with the ability to run the original Minecraft Server on the RasPi i was at first hoping that a small group of players could run on the original Java-based server software. With the OpenJDK and the Cocoa VM it indeed did start up.
Sadly the memory footprint was somewhat higher than i did assume from my previous tests on the Intel platform. So even connecting only one player was a nightmare and is no joy and far from playable. Memory swapping, high load and even the erratic process killing by the Linux kernel happened, when all memory (real + swap) is used up by the minecraft process and the system.
Besides the memory consumption it still is questionable if the CPU can run the Java implementation of the Minecraft server. Considering my following experience i doubt this!
So i ditched the idea of the original minecraft server and headed towards the alternative Minecraft server implementations (even while i am still hoping that the foundation might announce an 512MB or even 1GB variant of the RasPi )
It was already mentioned in this forum and after digging around in the huge wilderness of unfinished and stale Minecraft server implementations i reapproached the MC-Server
variant. It still is in active development and was a promising approach.
Other alternative Minecraft server implementation have been and still may be around. All i could find out about them was that they use other high-resource-intensive and inappropriate technologies (.net and other interpreted languages) or that their development got fall asleep. Some even got bought up by Mojang, the makers of Minecraft. So maybe there is hope for an less resource intensive official Minecraft server in the near future. The current development versions of the official Minecraft client already do integrate a server as a base for the single-player game... let's see!
On the official Debian Wheezy image it did run a lot faster than the original Minecraft server, mostly based on the fact that the MC-Server is implemented in C++ instead of Java and has a lot lower resource consumption. Still this server can reach the limits of the RasPi very quickly. But at least it was possible to connect one Minecraft client to it, even if (sadly) still not really playable. The high CPU constraint of the MC-Server was exhausting the joy this time.
But there was another optimization on the horizon... the Rasbian images showed up. Really promising because there is a lot of floating point stuff in an 3D game server going on, which this optimized Debian distribution is aiming at. So when my spare time made it possible and after the images were officially announced, i switched on one of my Raspis to this distribution and gave it another try.
The result is amazing. The startup time reduced significantly (from 25 seconds to 15 iirc). This time included loading the previously generated base level as well, which did not get mentioned in the other threads about this variant of the minecraft server. Creating and populating the Minecraft world took about two minutes when i started the MC-Server for the first time on the previously used Debian Wheezy image (armel).
Comprehensive tests to compare the performance may be done, but as the old (armel) Debian image was no joy at all, this would be waste of time.
Now i can even connect to the server using an up-to-date vanilla Minecraft client (version 1.2.5). Yesterday evening i did play around solo on the server and problems i did encounter was a crash of the MC-Server whie i had dinner. Currently i do compile the MC-Server after updating it to the current (trunk) version from their SVN repository.
The diffs from the update show significant changes in the implementation of the game logic. There are stil missing Minecraft features and supposedly bugs, but at least our RasPis now can host a server for a serious and fun game!
If there is enough interest in this topic we should forward the status to the MC-Server developers. Maybe we can push and support their project this way.
Of course there is the question about legality regarding MC-Server. I'd appreciate a professional answer regarding this. Please do not give amateurish opinions, i could do this myself. All i see is that the MC-Server setting i will use, does check the account validity of the connecting Minecraft client against Mojang's authenticating web servers. Be aware to not use stolen software, as that definitively is a crime.