Date: Wed, 9 Sep 2015 16:42:09 -0400
From: Chris Mason <clm@fb.com>
To: Andy Lutomirski <luto@amacapital.net>
CC: "Darrick J. Wong" <darrick.wong@oracle.com>,
        =?iso-8859-1?Q?P=E1draig?= Brady <P@draigbrady.com>,
        Anna Schumaker <Anna.Schumaker@netapp.com>,
        <linux-nfs@vger.kernel.org>,
        Linux btrfs Developers List <linux-btrfs@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>, Zach Brown <zab@zabbo.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>, <andros@netapp.com>,
        Christoph Hellwig <hch@infradead.org>, Coreutils <coreutils@gnu.org>
Subject: Re: [PATCH v1 0/8] VFS: In-kernel copy system call
Message-ID: <20150909204209.GF9511@ret.masoncoding.com>
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On Wed, Sep 09, 2015 at 01:37:44PM -0700, Andy Lutomirski wrote:
> On Wed, Sep 9, 2015 at 1:09 PM, Chris Mason <clm@fb.com> 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 <darrick.wong@oracle.com> 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.

Oh ok, we can do that pretty quickly with the btrfs searching ioctl
(just walk the items, really fast), but that's root only.

For a real interface maybe a btrfs specific ioctl to compare file
ranges.

-chris