Re: BTRFS file clone support for cp

[Joel Becker]

On Wed, Jul 29, 2009 at 07:14:37PM +0100, Pádraig Brady wrote:
> Chris Mason wrote:
> > On Wed, Jul 29, 2009 at 03:14:49PM +0100, Pádraig Brady wrote:
> >>
> >> 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.
> 
> Right. Well we can turn off this cloning by doing --sparse={never,always}
> but that has side effects. If we need an option then maybe we should have
> it turn on cloning rather than restore default cp behaviour?
> The side effects I thought of earlier, of COW without corresponding allocation
> were possible fragmentation on write or unexpected/mishandled ENOSPC.
> Also for endangered mechanical disks, subsequent processing could
> be slowed as the head seeks between the old and new data to be copied.
> Perhaps these are a small price to pay, especially considering that
> solid state disks will only be affected by the write()=ENOSPC issue.
> 
> At the moment we have these linking options:
> 
> cp -l, --link #for hardlinks
> cp -s, --symbolic-link #for symlinks
> 
> So perhaps we should support:
> 
> cp --link={soft,hard,cow}
> for symlink(), link() and reflink() respectively?
> I.E. link to the name, inode or extents respectively.

        I've cooked up 'ln -r' for reflinks, which works for ln(1) but
not for cp(1).  I have a git tree with the (in-flux) code on
oss.oracle.com:

[View]
http://oss.oracle.com/git/?p=jlbec/reflink.git;a=summary
[Pull]
git://oss.oracle.com/git/jlbec/reflink.git master

        This repository isn't designed to be an authorative patch for
coreutils.  Instead it provides a reflink(1) program that is actually ln
-r in disguise.  Later work would be to get coreutils updated
"properly".

Joel

-- 

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 This is the end, my only friend the end
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Joel Becker
Principal Software Developer
Oracle
E-mail: address@hidden
Phone: (650) 506-8127