> 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?
