Re: New copyfile system call - discuss before LSF?

[Ric Wheeler]

On 02/25/2013 04:14 PM, Andy Lutomirski wrote:
> On 02/21/2013 02:24 PM, Zach Brown wrote:
> > 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.
> >
> > 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.
> I thought the first thing people would ask for is to atomically create a
> new file and copy the old file into it (at least on local file systems).
>   The idea is that nothing should see an empty destination file, either
> by race or by crash.  (This feature would perhaps be described as a
> pony, but it should be implementable.)
>
> This would be like a better link(2).
>
> --Andy

Why would this need to be atomic? That would seem to be a very difficult 
property to provide across all target types with multi-GB sized files...

Ric


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