[Libguestfs] [PATCH] v2v: Optimize convert for images with small holes

[Nir Soffer]

"qemu-img convert" detects zeroes in allocated areas and punch holes in
the destination image. This may save space on the destination image, but
slows down conversion when using outputs such as rhv-upload, which have
very large overhead per requests.

Using the -S flag, we can treat small areas filled with zeroes as data,
limiting the number of requests, and speeding the operation.

Here is an example converting Fedora 30 image:

$ virt-builder fedora-30 -o src.img
...

$ qemu-img map -f raw --output json src.img | wc -l
213

$ qemu-img convert -f raw -O raw -t none -T none src.img dst.img

$ qemu-img map -f raw --output json dst.img | wc -l
1443

$ ls -lhs *.img
1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov  1 21:48 dst.img
1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov  1 21:46 src.img

Qemu did 1443 writes instead of 213 (5.8X). Lets repeat this conversion
with the -S option:

$ qemu-img convert -f raw -O raw -t none -T none -S 64k src.img dst.img

$ qemu-img map -f raw --output json dst.img | wc -l
213

$ ls -lhs *.img
1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov  1 21:48 dst.img
1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov  1 21:46 src.img

Picking a good value for -S is not easy. Testing show that 64k is best
value for this test image for limiting the number of requests:

$ for size in 4k 8k 16k 32k 64k; do \
    printf "%5s: " $size; \
    qemu-img convert -f raw -O raw -t none -T none -S $size src.img dst.img; \
    qemu-img map -f raw --output json dst.img | wc -l; \
done
   4k: 1443
   8k: 731
  16k: 521
  32k: 387
  64k: 213

We need more testing with oVirt to measure the performance improvement
and pick a good value. This should probably be an option, but lets start
with a minimal change.
---

Untested. I cannot build virt-v2v on Fedora 29 since libguestfs 1.41.5
is required but not available. I can also not build libguestfs from git
becuase setting up common submodule in ./autogen.sh fails.

 v2v/v2v.ml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/v2v/v2v.ml b/v2v/v2v.ml
index 4655b883..03590c9e 100644
--- a/v2v/v2v.ml
+++ b/v2v/v2v.ml
@@ -741,6 +741,7 @@ and copy_targets cmdline targets input output =
         (if not (quiet ()) then [ "-p" ] else []) @
         [ "-n"; "-f"; "qcow2"; "-O"; t.target_format ] @
         (if cmdline.compressed then [ "-c" ] else []) @
+        [ "-S"; "64k" ] @
         [ overlay_file; filename ] in
       let start_time = gettimeofday () in
       if run_command cmd <> 0 then
-- 
2.20.1