> 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 ? -- Chuck Lever
