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

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Message-ID: <CAJfpegvvN_5c5oMv8UoODXQHc-DQnijhOtPDXmNamVpQLDoWMQ@mail.gmail.com>
Date:	Mon, 30 Sep 2013 17:57:41 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	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>,
	"Schumaker, Bryan" <Bryan.Schumaker@...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 Mon, Sep 30, 2013 at 4:49 PM, Ric Wheeler <rwheeler@...hat.com> wrote:
> On 09/30/2013 10:46 AM, Miklos Szeredi wrote:
>>
>> On Mon, Sep 30, 2013 at 4:41 PM, Ric Wheeler <rwheeler@...hat.com> wrote:
>>>
>>> The way the array based offload (and some software side reflink works) is
>>> not a byte by byte copy. We cannot assume that a valid count can be
>>> returned
>>> or that such a count would be an indication of a sequential segment of
>>> good
>>> data.  The whole thing would normally have to be reissued.
>>>
>>> To make that a true assumption, you would have to mandate that in each of
>>> the specifications (and sw targets)...
>>
>> You're missing my point.
>>
>>   - user issues SIZE_MAX splice request
>>   - fs issues *64M* (or whatever) request to offload
>>   - when that completes *fully* then we return 64M to userspace
>>   - if it completes partially, then we return an error to userspace
>>
>> Again, wouldn't that work?
>>
>> Thanks,
>> Miklos
>
>
> Yes, if you send a copy offload command and it works, you can assume that it
> worked fully. It would be pretty interesting if that were not true :)
>
> If it fails, we cannot assume anything about partial completion.

Sure, that was my understanding from the start.  Maybe I wasn't
precise enough in my explanation.

Thanks,
Miklos
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