> SEEK_HOLE could provide a very simple heuristic to detect preallocated
> files (unfortunately ext4 does not implement SEEK_HOLE yet).

SEEK_HOLE is a weird (confusing and no intuitive) API (IMO).  There is
FIEMAP or whatever it's called which seems somewhat saner.

XFS specific there is also GETBMAP[X].


Simplest still compare the blocks allocated by the file
to it's length (ie. stat.st_blocks != stat.st_size>>9).