Re: BTRFS file clone support for cp

[Chris Mason]

On Wed, Jul 29, 2009 at 03:14:49PM +0100, Pádraig Brady wrote:
> Chris Mason wrote:
> > On Tue, Jul 28, 2009 at 10:06:35PM +0200, Giuseppe Scrivano wrote:
> >>
> >> I can't replicate it now, all tests I am doing report that blocks used
> >> before and after the clone are the same.  Probably yesterday the
> >> difference I noticed was in reality the original file flushed to the
> >> disk.
> > 
> > The clone will use some additional space for the metadata required to
> > point to the cloned blocks.  It isn't exactly O(1) it is O(metadata for
> > the file).
> 
> Thanks for the clarification Chris.
> So the just committed change in cp will
> link the destination file to the extents of the source.
> 
> We may need to play around with fallocate()
> if we want to get back to the original
> cp semantics of actually allocating space
> on the file system for the new file.

Well, best to just use the original cp code.  I was talking with
Giuseppe about this as well, I think we should the option to do regular
cp via a flag.

There will soon be a reflink system call that can be used on ocfs2 and
btrfs as well.  Thanks for adding this to glibc!

-chris