linux-kernel - Re: [RFC] extending splice for copy offloading

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Date:	Thu, 26 Sep 2013 20:06:41 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Zach Brown <zab@...hat.com>,
	Anna Schumaker <schumaker.anna@...il.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Bryan Schumaker <bjschuma@...app.com>,
	"Martin K. Petersen" <mkp@....net>, Jens Axboe <axboe@...nel.dk>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Eric Wong <normalperson@...t.net>
Subject: Re: [RFC] extending splice for copy offloading

On Thu, Sep 26, 2013 at 5:34 PM, J. Bruce Fields <bfields@...ldses.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@...hat.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.  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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