On Wed, Mar 08, 2017 at 03:00:52PM -0500, Olga Kornievskaia wrote: > > > On Mar 8, 2017, at 2:53 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > On Wed, Mar 08, 2017 at 12:32:12PM -0500, Olga Kornievskaia wrote: > >> > >>> On Mar 8, 2017, at 12:25 PM, Christoph Hellwig <hch@xxxxxxxxxxxxx> > >>> wrote: > >>> > >>> On Wed, Mar 08, 2017 at 12:05:21PM -0500, J. Bruce Fields wrote: > >>>> Since copy isn't atomic that check is never going to be reliable. > >>> > >>> That's true for everything that COPY does. By that logic we should > >>> not implement it at all (a logic that I'd fully support) > >> > >> If you were to only keep CLONE then you’d lose a huge performance gain > >> you get from server-to-server COPY. > > > > Yes. Also, I think copy-like copy implementations have reasonable > > semantics that are basically the same as read: > > > > - copy can return successfully with less copied than requested. > > - it's fine for the copied range to start and/or end past end of > > file, it'll just return a short read. > > - A copy of more than 0 bytes returning 0 means you're at end of > > file. > > > > The particular problem here is that that doesn't fit how clone works at > > all. > > > > It feels like what happened is that copy_file_range() was made mainly > > for the clone case, with the idea that copy might be reluctantly > > accepted as a second-class implementation. > > > > But the performance gain of copy offload is too big to just ignore, and > > in fact it's what copy_file_range does on every filesystem but btrfs and > > ocfs2 (and maybe cifs?), so I don't think we can just ignore it. > > > > If we had separate copy_file_range and clone_file_range, I *think* it > > could all be made sensible. Am I missing something? > > How would the application (cp) know when to call the clone_file_range and when to call copy_file_range? Try clone and then fall back on copy if that's not available? Which is the same thing vfs_copy_file_range() is doing now, but it'd seem less confusing if that logic was in the application. --b.
