Re: [RFC] extending splice for copy offloading

[Ric Wheeler <rwheeler@redhat.com>]

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.

Ric