Re: [RFC] extending splice for copy offloading

From: Zach Brown
Date: Thu Sep 26 2013 - 15:06:54 EST

On Thu, Sep 26, 2013 at 08:06:41PM +0200, Miklos Szeredi wrote:
> On Thu, Sep 26, 2013 at 5:34 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> 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@xxxxxxxxxx> 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. And for that perhaps we need a
> different API, one which has been discussed some time ago:
> asynchronous copyfile() returns immediately with a pollable event
> descriptor indicating copy progress, and some way to cancel the copy.
> And that can internally rely on ->direct_splice(), with appropriate
> algorithms for determine the optimal chunk size.

And perhaps we don't. Perhaps we can provide this much simpler
data-plane interface that works well enough for most everyone and can
avoid going down the async rat hole, yet again.

- z
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