Message-ID: <5244A5E7.90808@redhat.com>
Date: Thu, 26 Sep 2013 17:23:51 -0400
From: Ric Wheeler <rwheeler@redhat.com>
MIME-Version: 1.0
To: Miklos Szeredi <miklos@szeredi.hu>
CC: Zach Brown <zab@redhat.com>, "J. Bruce Fields" <bfields@fieldses.org>,
        Anna Schumaker <schumaker.anna@gmail.com>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        Trond Myklebust <Trond.Myklebust@netapp.com>,
        Bryan Schumaker <bjschuma@netapp.com>,
        "Martin K. Petersen" <mkp@mkp.net>, Jens Axboe <axboe@kernel.dk>,
        Mark Fasheh <mfasheh@suse.com>, Joel Becker <jlbec@evilplan.org>,
        Eric Wong <normalperson@yhbt.net>
Subject: Re: [RFC] extending splice for copy offloading
References: <1378919210-10372-1-git-send-email-zab@redhat.com> <CAELBmZBGD4rph=gjLCPKCdEj+nzEQ-F=DExoL+h3vRm7qF7dCQ@mail.gmail.com> <20130925183828.GA30372@lenny.home.zabbo.net> <CAFX2JfnyF8kyMYzCdqdr2JkoyQCom1bFLpFj89wODjoju54-Ow@mail.gmail.com> <20130925190620.GB30372@lenny.home.zabbo.net> <20130925195526.GA18971@fieldses.org> <20130925210742.GG30372@lenny.home.zabbo.net> <CAJfpegsQ0A3T+46o9nsPwaH83JCbgyhgRNGPgzTqs0EcsmDuiQ@mail.gmail.com> <20130926153359.GE704@fieldses.org> <CAJfpegsUchb0eX+Hi3rN5Ypje3Y-dgo=pxgM1Y3BQbHVp=1hSw@mail.gmail.com> <20130926190611.GP30372@lenny.home.zabbo.net> <CAJfpegvvWhs+jv2J9kOQrB31PEO3kyn_sLm_e2w9YKp=y6EDhA@mail.gmail.com>
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On 09/26/2013 03:53 PM, Miklos Szeredi wrote:
> On Thu, Sep 26, 2013 at 9:06 PM, Zach Brown <zab@redhat.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