[Libguestfs] [PATCH libnbd 3/3] copy: Do not initialize read buffer

[Richard W.M. Jones]

On Sun, Mar 06, 2022 at 10:27:30PM +0200, Nir Soffer wrote:
> nbdcopy checks pread error now, so we will never leak uninitialized data
> from the heap to the destination server. Testing show 3-8% speedup when
> copying a real image.
> 
> On laptop with 12 cores and 2 consumer NVMe drives:
> 
> $ qemu-nbd --read-only --persistent --shared 8 --cache none --aio native \
>     --socket /tmp/src.sock --format raw fedora-35-data.raw &
> 
> $ qemu-nbd --persistent --shared 8 --cache none --aio native --discard unmap \
>     --socket /tmp/dst.sock --format raw dst.raw &
> 
> $ hyperfine -p "sleep 5" "nbdcopy $SRC $DST" ".libs/nbdcopy $SRC $DST"
> 
> Benchmark 1: nbdcopy nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock
>   Time (mean ± σ):      2.065 s ±  0.057 s    [User: 0.296 s, System: 1.414 s]
>   Range (min … max):    2.000 s …  2.163 s    10 runs
> 
> Benchmark 2: .libs/nbdcopy nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock
>   Time (mean ± σ):      1.911 s ±  0.050 s    [User: 0.059 s, System: 1.544 s]
>   Range (min … max):    1.827 s …  1.980 s    10 runs
> 
> Summary
>   '.libs/nbdcopy nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock' ran
>     1.08 ± 0.04 times faster than 'nbdcopy nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock'
> 
> On server with 80 cores and a fast NVMe drive:
> 
> $ hyperfine "./nbdcopy-init $SRC $DST" "./nbdcopy-no-init $SRC $DST"
> 
> Benchmark 1: ./nbdcopy-init nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock
>   Time (mean ± σ):      2.652 s ±  0.033 s    [User: 0.345 s, System: 1.306 s]
>   Range (min … max):    2.619 s …  2.709 s    10 runs
> 
> Benchmark 2: ./nbdcopy-no-init nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock
>   Time (mean ± σ):      2.572 s ±  0.031 s    [User: 0.055 s, System: 1.409 s]
>   Range (min … max):    2.537 s …  2.629 s    10 runs
> 
> Summary
>   './nbdcopy-no-init nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock' ran
>     1.03 ± 0.02 times faster than './nbdcopy-init nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock'
> 
> Signed-off-by: Nir Soffer <nsoffer at redhat.com>
> ---
>  copy/nbd-ops.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/copy/nbd-ops.c b/copy/nbd-ops.c
> index adfe4de5..1218d6d0 100644
> --- a/copy/nbd-ops.c
> +++ b/copy/nbd-ops.c
> @@ -52,20 +52,21 @@ static void
>  open_one_nbd_handle (struct rw_nbd *rwn)
>  {
>    struct nbd_handle *nbd;
>  
>    nbd = nbd_create ();
>    if (nbd == NULL) {
>      fprintf (stderr, "%s: %s\n", prog, nbd_get_error ());
>      exit (EXIT_FAILURE);
>    }
>  
> +  nbd_set_pread_initialize (nbd, false);

I feel this is worth a small comment too even though it's not an
example.  Something simple like this is enough:

+  /* This is only safe because we always check for errors from pread. */
+  nbd_set_pread_initialize (nbd, false);

ACK series - with comments added.

Rich.

-- 
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