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

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Date:	Thu, 26 Sep 2013 17:23:51 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	Zach Brown <zab@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	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 09/26/2013 03:53 PM, Miklos Szeredi wrote:
> On Thu, Sep 26, 2013 at 9:06 PM, Zach Brown <zab@...hat.com> wrote:
>
>>> 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.
> I think either buffering or async is needed to get good perforrmace
> without too much complexity in the app (which is not good).  Buffering
> works quite well for regular I/O, so maybe its the way to go here as
> well.
>
> Thanks,
> Miklos
>

Buffering  misses the whole point of the copy offload - the idea is *not* to 
read or write the actual data in the most interesting cases which offload the 
operation to a smart target device or file system.

Regards,

Ric

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