[hachari]

I don’t see how read only helps at all.

Btrfs can dedupe at the block level.

[_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..

[fsiefken]

A compression benchmark of both filesystems would be of interest in this regard (lzo, zstd and zlib), both read speed and compression wise

[TimTheTinker]

Sure, but it sounds like block deduping is only one of several optimizations that DwarFS is able to take advantage of because it’s a read-only FS.