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"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.
(cherry picked from commit 2aa78ade2d48e926b7b04050338ebd8a0c5e3f05
in virt-v2v)
Libguestfs is tools and a library for accessing and modifying guest disk images. For more information see the home page: http://libguestfs.org/ For discussion, development, patches, etc. please use the mailing list: http://www.redhat.com/mailman/listinfo/libguestfs To find out how to build libguestfs from source, read: docs/guestfs-building.pod http://libguestfs.org/guestfs-building.1.html man docs/guestfs-building.1 Copyright (C) 2009-2019 Red Hat Inc. The library is distributed under the LGPLv2+. The programs are distributed under the GPLv2+. Please see the files COPYING and COPYING.LIB for full license information. The examples are under a very liberal license.