Re: New copyfile system call - discuss before LSF?

[Andy Lutomirski]

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

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.

--Andy
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