Ok, so the way I understand it is that when you do an upgrade it puts the previous fs state into a 'previous' subdirectory of each dfs.name.dir. This way the upgrade can be rolled back if something goes wrong (by basically doing a rename(prevDir, curDir)). When the upgrade is proven successful you can finalize the upgrade--which removes the prevDir--or rollback, which does the rename I mentioned. For this reason, starting a new upgrade checks to make sure the prevDir does not exist, for you would otherwise lose the ability to rollback a previously unfinished upgrade. Based on all of that and your exception, it seems as though an upgrade was or is already in progress and was not finalized. Have you run this upgrade command a few times? If so, do you have the output from the first run ever? All state is kept within these directories, so you should be able to take a manual backup like I mentioned and then try some things out. For instance you might want to try finalizing or rollback the previous upgrade and go from there. It would be nice if someone else chimed in because while I have somewhat of an understanding of how this works it is all based on my own reading of docs and the code, and not any real-world experience (like I said, I've never had to do an fs upgrade because we always migrate to a new cluster). Hope this helps.
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