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@xxxxxxxxxx www.netapp.com -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
