Release 2.0 upgrades must handle blocks being written to (bbw) files from 1.0 release. Problem reported by @brahmareddy
The DataNode will only have one block pool after upgrading from a 1.x release. (This is because in the 1.x releases, there were no block pools-- or equivalently, everything was in the same block pool). During the upgrade, we should hardlink the block files from the blocksBeingWritten directory into the rbw directory of this block pool. Similarly, on -finalize, we should delete the blocksBeingWritten directory.