"Myklebust, Trond" <Trond.Myklebust@xxxxxxxxxx> wrote: > > -----Original Message----- > > From: Zach Brown [mailto:zab@xxxxxxxxxx] > > Sent: Thursday, February 21, 2013 5:25 PM > > To: Myklebust, Trond > > Cc: Paolo Bonzini; Ric Wheeler; Linux FS Devel; linux-kernel@xxxxxxxxxxxxxxx; > > Chris L. Mason; Christoph Hellwig; Alexander Viro; Martin K. Petersen; > > Hannes Reinecke; Joel Becker > > Subject: Re: New copyfile system call - discuss before LSF? > > > > 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. > > psendfile() ? > > I fully agree that sounds reasonable... Just being an ass. :-) splice() already has offset for both fds and a flags arg: ssize_t splice(int fd_in, loff_t *off_in, int fd_out, loff_t *off_out, size_t len, unsigned int flags); The current downside is it requires one fd to be a pipe, so it's just not very easy to use from my perspective[1]. > > 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. > > vpsendfile() then? I agree that might be a little more future-proof. Particularly given that the underlying protocols tend to be fully asynchronous, and so it makes sense to queue up more than one copy at a time... splicev() might be nice to have in that case, too. [1] my splice() annoyances: * need to create/manage a pipe * copy size limited by pipe size * doesn't reduce userspace syscalls (just data copy overhead) * easy to misuse and starve with blocking sockets + big buffers * not many users, so bugs creep in (v3.7.8 was the first usable version of the 3.7 series for TCP sockets) -- 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
