[Libguestfs] [PATCH 5/6] v2v: Use unordered writes in qemu-img convert

[Nir Soffer]

Without unordered writes, qemu-img read data in parallel, but issue
requests in order, keeping only one in-flight request. With unordered
writes, qemu keeps up to 8 in-flight requests by default.

With this change we see significant improvement in the
rhv-upload-plugin. Connection time decreased from 7.08 seconds to 3.77
seconds, 88% faster. Compared with last version using single connection,
we are now 42% faster.

[connection 1 ops, 3.778596 s]
[dispatch 547 ops, 2.977509 s]
[write 464 ops, 2.703108 s, 355.62 MiB, 131.56 MiB/s]
[zero 81 ops, 0.118312 s, 1.22 GiB, 10.31 GiB/s]
[flush 2 ops, 0.000222 s]

[connection 1 ops, 3.774985 s]
[dispatch 555 ops, 2.976595 s]
[write 490 ops, 2.685608 s, 352.19 MiB, 131.14 MiB/s]
[zero 63 ops, 0.122802 s, 1.20 GiB, 9.80 GiB/s]
[flush 2 ops, 0.000203 s]

[connection 1 ops, 3.777071 s]
[dispatch 564 ops, 2.930730 s]
[write 499 ops, 2.631180 s, 323.38 MiB, 122.90 MiB/s]
[zero 63 ops, 0.119163 s, 1.17 GiB, 9.86 GiB/s]
[flush 2 ops, 0.000255 s]

[connection 1 ops, 3.778360 s]
[dispatch 528 ops, 2.979062 s]
[write 456 ops, 2.676340 s, 318.00 MiB, 118.82 MiB/s]
[zero 70 ops, 0.118221 s, 1.08 GiB, 9.18 GiB/s]
[flush 2 ops, 0.000202 s]

Signed-off-by: Nir Soffer <nsoffer at redhat.com>
---
 v2v/v2v.ml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/v2v/v2v.ml b/v2v/v2v.ml
index 54f26297..aa9d9713 100644
--- a/v2v/v2v.ml
+++ b/v2v/v2v.ml
@@ -763,6 +763,7 @@ and copy_targets cmdline targets input output =
         [ "-n"; "-f"; "qcow2"; "-O"; output#transfer_format t ] @
         (if cmdline.compressed then [ "-c" ] else []) @
         [ "-S"; "64k" ] @
+        [ "-W"; ] @
         [ overlay_file; filename ] in
       let start_time = gettimeofday () in
       if run_command cmd <> 0 then
-- 
2.26.2