Hmm, this discussion seems to assume that we only use
copy_file_range() to copy/clone whole segment files, right? That's
great and may even get most of the available benefit given typical
databases with many segments of old data that never changes, but... I
think copy_write_range() allows us to go further than the other
whole-file clone techniques: we can stitch together parts of an old
backup segment file and an incremental backup to create a new file.
If you're interested in minimising disk use while also removing
dependencies on the preceding chain of backups, then it might make
sense to do that even if you *also* have to read the data to compute
the checksums, I think? That's why I mentioned it: if
copy_file_range() (ie sub-file-level block sharing) is a solution in
search of a problem, has the world ever seen a better problem than
pg_combinebackup?