lkml.org 
[lkml]   [2013]   [Feb]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: New copyfile system call - discuss before LSF?
    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




    \
     
     \ /
      Last update: 2013-02-25 23:23    [W:3.683 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site