Re: New copyfile system call - discuss before LSF?

[Myklebust, Trond]

On Mon, 2013-02-25 at 15:35 -0800, Andy Lutomirski wrote:
> On Mon, Feb 25, 2013 at 3:28 PM, Myklebust, Trond
> <Trond.Myklebust@xxxxxxxxxx> wrote:
> > On Mon, 2013-02-25 at 14:16 -0800, Andy Lutomirski wrote:
> >> On Mon, Feb 25, 2013 at 1:59 PM, Myklebust, Trond
> >> <Trond.Myklebust@xxxxxxxxxx> wrote:
> >> > 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?
> >> >
> >>
> >> Download file from some source and then verify it.  Now copyfile it
> >> into my repository of known-good files.
> >>
> >> Admittedly I could link + unlink or rename it there, but I consider
> >> hard links to be rather evil, especially when cow links are available.
> >
> > Rename is the right way to do that as it can't corrupt the data after
> > you have verified it. copyfile can...
> 
> ...copyfile doesn't exist.

Wrong! The underlying NFS and SCSI copy offload protocols are fully
defined at this time, and will constrain any implementation that you may
dream up.

>   I think it would be neat if it couldn't
> corrupt data.

It would also be neat if the moon were made of cheese... The underlying
NFS and SCSI protocols do not guarantee perfect copies; the copy may,
for instance, be interrupted due to external circumstances.

> In any case, this may be a bad idea -- presumably you'd have to fsync
> the file you're copying *from* first to avoid a massive performance
> hit.

You have to do that anyway.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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