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

[Nir Soffer]

On Fri, Nov 1, 2019, 22:56 Nir Soffer <nirsof at gmail.com> wrote:

> "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
>

I finally built this by applying the patch to libguestfs 1.40.1.

I did initial testing with local output like this:

$ ./run virt-v2v -v -x -i disk /home/nsoffer/tmp/sparse-size/src.img -o
local -of raw -os /home/nsoffer/tmp/sparse-size

Testing show that this change improves throughput, but converting with
local file system is so fast that this is not interesting.

I hope to more interesting results with rhv-upload. Hopefully I can test
this next week.

---

With 1.40.1:

qemu-img 'convert' '-p' '-n' '-f' 'qcow2' '-O' 'raw'
'/home/nsoffer/src/libguestfs/tmp/v2vovld6b8b9.qcow2'
'/home/nsoffer/tmp/sparse-size/src-sda'
    (100.00/100%)
du --block-size=1 '/home/nsoffer/tmp/sparse-size/src-sda' | awk '{print $1}'
virtual copying rate: 59718.8 M bits/sec
real copying rate: 11479.2 M bits/sec
sda: estimate 2456662831 (2.3G) versus actual 1238372352 (1.2G): 98.4%

qemu-img 'convert' '-p' '-n' '-f' 'qcow2' '-O' 'raw'
'/home/nsoffer/src/libguestfs/tmp/v2vovl70b9b6.qcow2'
'/home/nsoffer/tmp/sparse-size/src-sda'
    (100.00/100%)
du --block-size=1 '/home/nsoffer/tmp/sparse-size/src-sda' | awk '{print $1}'
virtual copying rate: 65311.3 M bits/sec
real copying rate: 12567.4 M bits/sec
sda: estimate 2456662831 (2.3G) versus actual 1239678976 (1.2G): 98.2%

qemu-img 'convert' '-p' '-n' '-f' 'qcow2' '-O' 'raw'
'/home/nsoffer/src/libguestfs/tmp/v2vovl1c8ada.qcow2'
'/home/nsoffer/tmp/sparse-size/src-sda'
    (100.00/100%)
du --block-size=1 '/home/nsoffer/tmp/sparse-size/src-sda' | awk '{print $1}'
virtual copying rate: 68175.6 M bits/sec
real copying rate: 13118.6 M bits/sec
sda: estimate 2456662831 (2.3G) versus actual 1239678976 (1.2G): 98.2%

With 1.40.1 + patch:

qemu-img 'convert' '-p' '-n' '-f' 'qcow2' '-O' 'raw' '-S' '64k'
'/home/nsoffer/src/libguestfs/tmp/v2vovl981419.qcow2'
'/home/nsoffer/tmp/sparse-size/src-sda'
    (100.00/100%)
du --block-size=1 '/home/nsoffer/tmp/sparse-size/src-sda' | awk '{print $1}'
virtual copying rate: 70717.5 M bits/sec
real copying rate: 13763.8 M bits/sec
sda: estimate 2456662831 (2.3G) versus actual 1253900288 (1.2G): 95.9%

qemu-img 'convert' '-p' '-n' '-f' 'qcow2' '-O' 'raw' '-S' '64k'
'/home/nsoffer/src/libguestfs/tmp/v2vovl10e7e3.qcow2'
'/home/nsoffer/tmp/sparse-size/src-sda'
    (100.00/100%)
du --block-size=1 '/home/nsoffer/tmp/sparse-size/src-sda' | awk '{print $1}'
virtual copying rate: 73361.8 M bits/sec
real copying rate: 14278.5 M bits/sec
sda: estimate 2456662831 (2.3G) versus actual 1253900288 (1.2G): 95.9%

qemu-img 'convert' '-p' '-n' '-f' 'qcow2' '-O' 'raw' '-S' '64k'
'/home/nsoffer/src/libguestfs/tmp/v2vovldf5aaf.qcow2'
'/home/nsoffer/tmp/sparse-size/src-sda'
    (100.00/100%)
du --block-size=1 '/home/nsoffer/tmp/sparse-size/src-sda' | awk '{print $1}'
virtual copying rate: 73106.5 M bits/sec
real copying rate: 14228.8 M bits/sec
sda: estimate 2456662831 (2.3G) versus actual 1253900288 (1.2G): 95.9%
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