Datanode corruption

Our admins accidentally started a second nutch datanode pointing to the same directories as one already running (same machine) which in turn caused the entire contents of the datanode to go disappear.

This happened because the blocking was based on the username (since fixed in our start scripts) and it was started as two different users.

The ndfs.name.dir and ndfs.data.dir directories were both completely devoid of content, where they had about 150GB not all that much earlier.

I think the solution is improved interlocking within the data directory itself (file locked with flock or something similar).