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. --Andy -- 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
