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

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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...

Cheers
   Trond
-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com

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