Re: [PATCH v1 0/8] VFS: In-kernel copy system call

[Andy Lutomirski]

On Wed, Sep 9, 2015 at 1:09 PM, Chris Mason <address@hidden> wrote:
> On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote:
>> On Tue, Sep 8, 2015 at 3:39 PM, Darrick J. Wong <address@hidden> wrote:
>> > On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:
>> >> What I meant by this was: if you ask for "regular copy", you may end
>> >> up with a reflink anyway.  Anyway, how can you reflink a range and
>> >> have the contents *not* be the same?
>> >
>> > reflink forcibly remaps fd_dest's range to fd_src's range.  If they didn't
>> > match before, they will afterwards.
>> >
>> > dedupe remaps fd_dest's range to fd_src's range only if they match, of 
>> > course.
>> >
>> > Perhaps I should have said "...if the contents are the same before the 
>> > call"?
>> >
>>
>> Oh, I see.
>>
>> Can we have a clean way to figure out whether two file ranges are the
>> same in a way that allows false negatives?  I.e. return 1 if the
>> ranges are reflinks of each other and 0 if not?  Pretty please?  I've
>> implemented that in the past on btrfs by syncing the ranges and then
>> comparing FIEMAP output, but that's hideous.
>
> I'd almost rather have a separate call, maybe unshare_file_range()?
>
> Is that the end goal to the sharing check?

My use case was archival.  I can reflink data between a working copy
and some archived copy and then I can very efficiently tell if the
working copy has been changed by checking if the reflink is still
linked.

It would be even better if I could enumerate which parts of one file
match which parts of another file.

--Andy