Re: New copyfile system call - discuss before LSF?

Myklebust, Trond
2013-02-25T21:59:42Z

On Mon, 2013-02-25 at 16:49 -0500, Ric Wheeler wrote:
> 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...

Right. It may sound cool, but what's the real-life use case?

--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com