This tool requires that (at least) the node being balanced should have the DataNode process down. It's probably best to take down the cluster to stop hadoop from trying to make sure block replica minimums are met.
How to run
This is built specifically against version 1.0.3 of hadoop. It
../hadoop-common will contain a built version of
hadoop. I'm not really good with Java so the
build.xml file is
kind of brittle this way.
The easiest way to run it is to just use
ant run -Dargs=/path/to/hadoop/config
Do that as the hadoop user and it will move the blocks around.
There is no dry run option. If it fails in the middle of moving a block, it will be unhappy.