Re: [RFC] extending splice for copy offloading

Rob Landley
2013-10-06T08:42:42Z

On 09/26/2013 01:06:41 PM, Miklos Szeredi wrote:
> On Thu, Sep 26, 2013 at 5:34 PM, J. Bruce Fields
> <bfields@fieldses.org> wrote:
> > On Thu, Sep 26, 2013 at 10:58:05AM +0200, Miklos Szeredi wrote:
> >> On Wed, Sep 25, 2013 at 11:07 PM, Zach Brown <zab@redhat.com>
> wrote:
> >> >> A client-side copy will be slower, but I guess it does have the
> >> >> advantage that the application can track progress to some
> degree, and
> >> >> abort it fairly quickly without leaving the file in a totally
> undefined
> >> >> state--and both might be useful if the copy's not a simple
> constant-time
> >> >> operation.
> >> >
> >> > I suppose, but can't the app achieve a nice middle ground by
> copying the
> >> > file in smaller syscalls? Avoid bulk data motion back to the
> client,
> >> > but still get notification every, I dunno, few hundred meg?
> >>
> >> Yes. And if "cp" could just be switched from a read+write syscall
> >> pair to a single splice syscall using the same buffer size.
> >
> > Will the various magic fs-specific copy operations become
> inefficient
> > when the range copied is too small?
>
> We could treat spice-copy operations just like write operations (can
> be buffered, coalesced, synced).
>
> But I'm not sure it's worth the effort; 99% of the use of this
> interface will be copying whole files.

My "patch" implementation (in busybox and toybox) hits a point where it
wants to copy the rest of the file, once there are no more hunks to
apply. This is not copying a whole file. A similar thing happens with
tail when you use the +N syntax to skip start instead of end lines. I
can see sed doing a similar thing when told to operate on line ranges...

Note sure your 99% holds up here.

Rob