[Ocfs2-devel] [PATCH 1/3] fs: Document the reflink(2) system call.

[Joel Becker]

On Tue, May 05, 2009 at 10:57:11PM +0100, Jamie Lokier wrote:
> jim owens wrote:
> > 3) the granularity of the COW (1-byte write may cause
> >    1-block up through whole file copy) is fs-dependent.
> 
> And yet ENOSYS if the fs cannot implement any COW, and it isn't
> possible for userspace to duplicate the semantics by explicit copying?

	The point-in-time of the snapshot is what's important here.

> Do we say anything about attribute changes triggering COW or not, or
> leave it fs-dependent?  Given 3) fs-dependent makes sense, but it's
> nice to know in advance if { reflink -R old_tree saved_tree; chmod -R
> a-w saved_tree } will be as expensive as copying or as cheap as linking.

	"Shares the data extents of the source file".  I should hope
that chmod doesn't require copying out all the data.

Joel

-- 

Life's Little Instruction Book #267

	"Lie on your back and look at the stars."

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127