Re: [Lsf-pc] [LSF/MM][ATTEND]filesystem -- reflink

[Dave Chinner <david@fromorbit.com>]

On Wed, Jan 15, 2014 at 10:03:12AM +0100, Jan Kara wrote:
>   Hello,
> 
> On Tue 14-01-14 23:16:20, mingming cao wrote:
> > I'd like to attend Linux storage and filesystem summit. I am
> > interested in discussion of general lockless direct io and more
> > interested in discussion of reflink support for filesystem. Btrfs
> > and OCFS2 has this support, My goal is explore what's best to way
> > this support for ext4. If there is any crossover work between
> > filesystem and vfs people, or filesystem and dm layer I am more than
> > happy to discuss about it too.
>   There was a project for implementing COW for ext4 but it was a really
> major surgery and in the end didn't get to an upstreamable state. Reflink
> is somewhat simpler than general COW because it's only about fs data. In
> particular implementing reflink with a file granularity (i.e., a type of
> hardlink which is automatically converted to a copy when first opened for
> writing) is relatively simple to do but I'm not sure how useful it is.
> Doing reflink properly with block granularity is harder with block
> refcounting etc.
> 
> Do you have any particular usecase in mind?

The use case I keep hearing for reflink in XFS is optimising
distributed filesystem snapshot implementations, which tend to
require file block level COW semantics on a per-file basis...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com