Re: [RFC] extending splice for copy offloading

[Miklos Szeredi]

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.

Thanks,
Miklos
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