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Date: Mon, 30 Sep 2013 17:57:41 +0200
Message-ID: <CAJfpegvvN_5c5oMv8UoODXQHc-DQnijhOtPDXmNamVpQLDoWMQ@mail.gmail.com>
Subject: Re: [RFC] extending splice for copy offloading
From: Miklos Szeredi <miklos@szeredi.hu>
To: Ric Wheeler <rwheeler@redhat.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>,
        "Myklebust, Trond" <Trond.Myklebust@netapp.com>,
        Zach Brown <zab@redhat.com>, 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>,
        "Schumaker, Bryan" <Bryan.Schumaker@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>
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Sender: linux-nfs-owner@vger.kernel.org

On Mon, Sep 30, 2013 at 4:49 PM, Ric Wheeler <rwheeler@redhat.com> wrote:
> On 09/30/2013 10:46 AM, Miklos Szeredi wrote:
>>
>> On Mon, Sep 30, 2013 at 4:41 PM, Ric Wheeler <rwheeler@redhat.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