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

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(a)redhat.com&gt; --- 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); nbd_set_debug (nbd, verbose); /* Set the handle name for debugging. We could use rwn->rw.name * here but it is usually set to the lengthy NBD URI * (eg. "nbd://localhost:10809") which makes debug messages very * long. */ if (verbose) { char *name; const size_t index = rwn->handles.len; -- 2.35.1