Re: copy offload support in Linux - new system call needed?

[Chris Mason <chris.mason@oracle.com>]

On Thu, Dec 15, 2011 at 09:59:00AM -0500, Trond Myklebust wrote:
> On Wed, 2011-12-14 at 17:27 -0500, J. Bruce Fields wrote: 
> > On Wed, Dec 14, 2011 at 02:42:38PM -0500, Ric Wheeler wrote:
> > > On 12/14/2011 02:27 PM, Al Viro wrote:
> > > >On Wed, Dec 14, 2011 at 02:22:07PM -0500, Ric Wheeler wrote:
> > > >
> > > >>We had an active thread a couple of years back that came out of the
> > > >>reflink work and, at the time, there seemed to be moderately
> > > >>positive support for adding a new system call that would fit this
> > > >>use case (Joel Becker's copyfile()).
> > > >>
> > > >>Can we resurrect this effort? Is copyfile() still a good way to go,
> > > >>or should we look at other hooks?
> > > >copyfile(2) is probably a good way to go, provided that we do _not_
> > > >go baroque as it had happened the last time syscall had been discussed.
> > > >
> > > >IOW, to hell with progress reports, etc. - just a fastpath kind of
> > > >thing, in the same kind of relationship to cp(1) as rename(2) is to mv(1).
> > > >If it works - fine, if not - caller has to be ready to deal with handling
> > > >cross-device case anyway.
> > > 
> > > I think that this approach makes a lot of sense. Most of the
> > > devices/targets that support the copy offload, will do it in very
> > > reasonable amounts of time.
> > 
> > The current NFSv4.2 draft rolls both the "fast" and "slow" cases into
> > one operation:
> > 
> > 	http://tools.ietf.org/html/draft-ietf-nfsv4-minorversion2-06#section-2
> >  
> > Perhaps we should ask for separate operations for the two cases.  (Or at
> > least a "please don't bother if this is going to take 8 hours" flag....)
> 
> How would the server know? I suggest we deal with this by adding an
> ioctl() to allow the application to poll for progress: I'm assuming now
> that we don't expect more than 1 copyfile() system call at a time per
> file descriptor...

If we're using this to copy VM image files, I could easily imagine
wanting to clone multiple copies of the VM in parallel.

-chris