Re: [RFC] extending splice for copy offloading

[Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>]

On 09/30/2013 06:31 PM, Miklos Szeredi wrote:
> Here's an example "cp" app using direct splice (and without fallback to
> non-splice, which is obviously required unless the kernel is known to support
> direct splice).
>
> Untested, but trivial enough...
>
> The important part is, I think, that the app must not assume that the kernel can
> complete the request in one go.
>
> Thanks,
> Miklos
>
> ----
> #define _GNU_SOURCE
>
> #include <stdio.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <limits.h>
> #include <sys/stat.h>
> #include <err.h>
>
> #ifndef SPLICE_F_DIRECT
> #define SPLICE_F_DIRECT        (0x10)  /* neither splice fd is a pipe */
> #endif
>
> int main(int argc, char *argv[])
> {
> 	struct stat stbuf;
> 	int in_fd;
> 	int out_fd;
> 	int res;
> 	off_t off;

off_t off = 0;

>
> 	if (argc != 3)
> 		errx(1, "usage: %s from to", argv[0]);
>
> 	in_fd = open(argv[1], O_RDONLY);
> 	if (in_fd == -1)
> 		err(1, "opening %s", argv[1]);
>
> 	res = fstat(in_fd, &stbuf);
> 	if (res == -1)
> 		err(1, "fstat");
>
> 	out_fd = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, stbuf.st_mode);
> 	if (out_fd == -1)
> 		err(1, "opening %s", argv[2]);
>
> 	do {
> 		off_t in_off = off, out_off = off;
> 		ssize_t rres;
>
> 		rres = splice(in_fd, &in_off, out_fd, &out_off, SSIZE_MAX,
> 			     SPLICE_F_DIRECT);
> 		if (rres == -1)
> 			err(1, "splice");
> 		if (rres == 0)
> 			break;
>
> 		off += rres;
> 	} while (off < stbuf.st_size);
>
> 	res = close(in_fd);
> 	if (res == -1)
> 		err(1, "close");
>
> 	res = fsync(out_fd);
> 	if (res == -1)
> 		err(1, "fsync");
>
> 	res = close(out_fd);
> 	if (res == -1)
> 		err(1, "close");
>
> 	return 0;
> }


It would be nice if there would be way if the file system would get a 
hint that the target file is supposed to be copy of another file. That 
way distributed file systems could also create the target-file with the 
correct meta-information (same storage targets as in-file has).
Well, if we cannot agree on that, file system with a custom protocol at 
least can detect from 0 to SSIZE_MAX and then reset metadata. I'm not 
sure if this would work for pNFS, though.


Bernd