linux-kernel - Re: New copyfile system call

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Date:	Thu, 21 Feb 2013 22:13:57 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Ric Wheeler <rwheeler@...hat.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Chris L. Mason" <clmason@...ionio.com>,
	Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <aviro@...hat.com>,
	"Martin K. Petersen" <mkp@....net>, Hannes Reinecke <hare@...e.de>,
	"Joel Becker" <jlbec@...lplan.org>
Subject: Re: New copyfile system call - discuss before LSF?

On Thu, 2013-02-21 at 23:05 +0100, Ric Wheeler wrote:
> On 02/21/2013 09:00 PM, Paolo Bonzini wrote:
> > Il 21/02/2013 15:57, Ric Wheeler ha scritto:
> >>> sendfile64() pretty much already has the right arguments for a
> >>> "copyfile", however it would be nice to add a 'flags' parameter: the
> >>> NFSv4.2 version would use that to specify whether or not to copy file
> >>> metadata.
> >> That would seem to be enough to me and has the advantage that it is an
> >> relatively obvious extension to something that is at least not totally
> >> unknown to developers.
> >>
> >> Do we need more than that for non-NFS paths I wonder? What does reflink
> >> need or the SCSI mechanism?
> > For virt we would like to be able to specify arbitrary block ranges.
> > Copying an entire file helps some copy operations like storage
> > migration.  However, it is not enough to convert the guest's offloaded
> > copies to host-side offloaded copies.
> >
> > Paolo
> 
> I don't think that the NFS protocol allows arbitrary ranges, but the SCSI 
> commands are ranged based.
> 
> If I remember what the windows people said at a SNIA event a few years back, 
> they have a requirement that the target file be pre-allocated (at least for the 
> SCSI based copy). Not clear to me where they iterate over that target file to do 
> the block range copies, but I suspect it is in their kernel.

The NFSv4.2 copy offload protocol _does_ allow the copying of arbitrary
byte ranges. The main target for that functionality is indeed
virtualisation and thin provisioning of virtual machines.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com
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