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 -- 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
