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From 8d39f6bcf21a7c76e29c270b9304c978bbf2e8ba Mon Sep 17 00:00:00 2001
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From: Nir Soffer <nirsof@gmail.com>
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Date: Fri, 1 Nov 2019 22:56:18 +0100
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Subject: [PATCH] v2v: Optimize convert for images with small holes
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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"qemu-img convert" detects zeroes in allocated areas and punch holes in
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the destination image. This may save space on the destination image, but
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slows down conversion when using outputs such as rhv-upload, which have
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very large overhead per requests.
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Using the -S flag, we can treat small areas filled with zeroes as data,
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limiting the number of requests, and speeding the operation.
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Here is an example converting Fedora 30 image:
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$ virt-builder fedora-30 -o src.img
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...
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$ qemu-img map -f raw --output json src.img | wc -l
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213
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$ qemu-img convert -f raw -O raw -t none -T none src.img dst.img
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$ qemu-img map -f raw --output json dst.img | wc -l
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1443
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$ ls -lhs *.img
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1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov 1 21:48 dst.img
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1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov 1 21:46 src.img
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Qemu did 1443 writes instead of 213 (5.8X). Lets repeat this conversion
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with the -S option:
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$ qemu-img convert -f raw -O raw -t none -T none -S 64k src.img dst.img
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$ qemu-img map -f raw --output json dst.img | wc -l
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213
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$ ls -lhs *.img
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1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov 1 21:48 dst.img
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1.2G -rw-r--r--. 1 nsoffer nsoffer 6.0G Nov 1 21:46 src.img
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Picking a good value for -S is not easy. Testing show that 64k is best
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value for this test image for limiting the number of requests:
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$ for size in 4k 8k 16k 32k 64k; do \
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printf "%5s: " $size; \
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qemu-img convert -f raw -O raw -t none -T none -S $size src.img dst.img; \
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qemu-img map -f raw --output json dst.img | wc -l; \
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done
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4k: 1443
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8k: 731
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16k: 521
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32k: 387
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64k: 213
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We need more testing with oVirt to measure the performance improvement
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and pick a good value. This should probably be an option, but lets start
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with a minimal change.
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(cherry picked from commit 2aa78ade2d48e926b7b04050338ebd8a0c5e3f05
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in virt-v2v)
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---
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v2v/v2v.ml | 1 +
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1 file changed, 1 insertion(+)
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diff --git a/v2v/v2v.ml b/v2v/v2v.ml
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index d7a868659..a81a2320a 100644
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--- a/v2v/v2v.ml
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+++ b/v2v/v2v.ml
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@@ -754,6 +754,7 @@ and copy_targets cmdline targets input output =
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(if not (quiet ()) then [ "-p" ] else []) @
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[ "-n"; "-f"; "qcow2"; "-O"; t.target_format ] @
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(if cmdline.compressed then [ "-c" ] else []) @
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+ [ "-S"; "64k" ] @
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[ overlay_file; filename ] in
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let start_time = gettimeofday () in
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if run_command cmd <> 0 then
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--
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2.26.2
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