Once you need to load any parts of a file into memory you might as well compute the hash for all the bytes that you've loaded into memory. The act of loading th

Once you need to load any parts of a file into memory you might as well compute the hash for all the bytes that you've loaded into memory. The act of loading the bytes from permanent storage (even NVME SSD) is likely to be many orders of magnitude slower than actually computing the hash. So it makes no sense to just "hash parts of the file" when in all likelihood the entire file will be getting loaded into memory even if you actively read only a few bytes from each 4k block.