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

On Wed, Sep 9, 2015 at 1:09 PM, Chris Mason [off-list ref] wrote:

On Tue, Sep 08, 2015 at 04:08:43PM -0700, Andy Lutomirski wrote:

quoted

On Tue, Sep 8, 2015 at 3:39 PM, Darrick J. Wong [off-list ref] wrote:

quoted

On Tue, Sep 08, 2015 at 02:45:39PM -0700, Andy Lutomirski wrote:

quoted

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
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