Re: [RFC] extending splice for copy offloading

Bernd Schubert
2013-09-30T17:29:47Z

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