On Fri, Sep 04, 2020 at 09:56:19AM -0400, Chuck Lever wrote: > > > > On Sep 4, 2020, at 9:52 AM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > On Tue, Sep 01, 2020 at 03:18:54PM -0400, J. Bruce Fields wrote: > >> On Tue, Sep 01, 2020 at 01:40:16PM -0400, Anna Schumaker wrote: > >>> On Tue, Sep 1, 2020 at 12:49 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > >>>> > >>>> On Mon, Aug 31, 2020 at 02:16:26PM -0400, Anna Schumaker wrote: > >>>>> On Fri, Aug 28, 2020 at 5:56 PM J. Bruce Fields <bfields@xxxxxxxxxx> wrote: > >>>>>> We really don't want to bother encoding small holes. I doubt > >>>>>> filesystems want to bother with them either. Do they give us any > >>>>>> guarantees as to the minimum size of a hole? > >>>>> > >>>>> The minimum size seems to be PAGE_SIZE from everything I've seen. > >>>> > >>>> OK, can we make that assumption explicit? It'd simplify stuff like > >>>> this. > >>> > >>> I'm okay with that, but it's technically up to the underlying filesystem. > >> > >> Maybe we should ask on linux-fsdevel. > >> > >> Maybe minimum hole length isn't the right question: suppose at time 1 a > >> file has a single hole at bytes 100-200, then it's modified so at time 2 > >> it has a hole at bytes 50-150. If you lseek(fd, 0, SEEK_HOLE) at time > >> 1, you'll get 100. Then if you lseek(fd, 100, SEEK_DATA) at time 2, > >> you'll get 150. So you'll encode a 50-byte hole in the READ_PLUS reply > >> even though the file never had a hole smaller than 100 bytes. > >> > >> Minimum hole alignment might be the right idea. > >> > >> If we can't get that: maybe just teach encode_read to stop when it > >> *either* returns maxcount worth of file data (and holes) *or* maxcount > >> of encoded xdr data, just to prevent a weird filesystem from triggering > >> a bug. > > > > Alternatively, if it's easier, we could enforce a minimum alignment by > > rounding up the result of SEEK_HOLE to the nearest multiple of (say) 512 > > bytes, and rounding down the result of SEEK_DATA. > > Perhaps it goes without saying, but is there an effort to > ensure that the set of holes is represented in exactly the > same way when accessing a file via READ_PLUS and > SEEK_DATA/HOLE ? So you're thinking of something like a pynfs test that creates a file with holes and then tries reading through it with READ_PLUS and SEEK and comparing the results? There are lots of legitimate reasons that test might "fail"--servers aren't required to support holes at all, and have a lot of lattitude about how to report them. But it might be a good idea to test anyway. --b.
