Can't start mongodb in ArchLinux
You ran sudo mkdir -p /data/db as not yourself.  Now that directory is owned by root.

You need to chown the directory back to yourself before starting mongod otherwise mongod cannot access the directory write the files into it that are needed.

Please also find a 64-bit platform to run mongo - it's not really a very good idea to run it on 32-bit system unless this is just for playing around with.