ZFS Deduplication on Existing Pool
Similarly to enabling compression on a ZFS partition that already has data, enabling deduplication does not deduplicate the data that already exists. This was a problem for me. To add to the problem, this pool had many partitions and the partitions each had many snapshots, all of which I wanted to preserve.
The only solution seems to be moving all the data away and back. I bought 4 external hard drives, made a storagebomb and away I went. This page has a good description of how to move an entire pool to a different device, preserving all the snapshots and whatnot.
