On Tue, Aug 30, 2011 at 11:29 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > On Tue, Aug 30, 2011 at 06:17:02PM -0700, Sunil Mushran wrote: >> Instead >> we should let the fs weigh the cost of providing accurate information >> with the possible gain in performance. >> >> Data: >> A range in a file that could contain something other than nulls. >> If in doubt, it is data. >> >> Hole: >> A range in a file that only contains nulls. > > And that's -exactly- the ambiguous, vague definition that has raised > all these questions in the first place. I was in doubt about whether > unwritten extents can be considered a hole, and by your definition > that means it should be data. But Andreas seems to be in no doubt it > should be considered a hole. That's fine, though. Different filesystems have different abilities to recognize a data hole - FAT can't do it at all. Perhaps the requirements would be better stated in reverse: If the filesystem knows that a read() will return nulls (for whatever reason based on it's internal knowledge), it can report a hole. If it can't guarantee that, it's data. It's an absolute requirement that SEEK_DATA never miss data. SEEK_HOLE working is a nicety that userspace would appreciate - remember that the consumer here is cp(1), using it to skip empty portions of files and create sparse destination files. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
