This is a new way to punch holes in files, but the concept of files with holes in them is far from new.
This leads to a couple obvious answers
> What sorts of controls over who can mark data like that will there be? Can it cause a security issue for data to disappear in the middle of a file unexpectedly? Maybe clearing volatile-ness on file ownership or permissions change fixes that?
Tie this to the ability to modify/truncate the file and you are not adding any new possibilities, just new ways to trigger the possibilities (someone who can modify the file can truncate it, write a new file missing some data, etc)
> How can I tell how much space is used by such data?
the same way you would find out how much space is used by other sparse files today.