RE: New copyfile system call - discuss before LSF?

Myklebust, Trond
2013-02-22T01:30:03Z

> -----Original Message-----
> From: Zach Brown [mailto:zab@redhat.com]
> Sent: Thursday, February 21, 2013 5:25 PM
> To: Myklebust, Trond
> Cc: Paolo Bonzini; Ric Wheeler; Linux FS Devel; linux-kernel@vger.kernel.org;
> Chris L. Mason; Christoph Hellwig; Alexander Viro; Martin K. Petersen;
> Hannes Reinecke; Joel Becker
> Subject: Re: New copyfile system call - discuss before LSF?
>
> On Thu, Feb 21, 2013 at 08:50:27PM +0000, Myklebust, Trond wrote:
> > On Thu, 2013-02-21 at 21:00 +0100, 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.
> >
> > So how would a system call based on sendfile64() plus my flag
> > parameter prevent an underlying implementation from meeting your
> criterion?
>
> If I'm guessing correctly, sendfile64()+flags would be annoying because it's
> missing an out_fd_offset. The host will want to offload the guest's copies by
> calling sendfile on block ranges of a guest disk image file that correspond to
> the mappings of the in and out files in the guest.
>
> You could make it work with some locking and out_fd seeking to set the
> write offset before calling sendfile64()+flags, but ugh.
>
> ssize_t sendfile(int out_fd, int in_fd, off_t in_offset, off_t
> out_offset, size_t count, int flags);
>
> That seems closer.

psendfile() ?

I fully agree that sounds reasonable... Just being an ass. :-)

> We might also want to pre-emptively offer iovs instead of offsets, because
> that's the very first thing that's going to be requested after people prototype
> having to iterate calling sendfile() for each contiguous copy region.

vpsendfile() then? I agree that might be a little more future-proof. Particularly given that the underlying protocols tend to be fully asynchronous, and so it makes sense to queue up more than one copy at a time...

Cheers,
Trond