[_flux]

Consider how much of work in btrfs is done to just handle the case of modifying existing files—or reducing file system size.. It is basically the reason it uses b-trees. It's in the name!

For example, when dedupping in block level it needs to know (right?) how many times a block is being used, so it can be collected when it runs out of uses.

ISO9660 can also express dedupped (hardlinked) files with the Rock Ridge extensions. I don't know but I'm wondering it could even do block-level dedupping if the generating program abused the format a bit..