ffa244c84a
Especially when O_DIRECT is used with image files so that the page cache
indirection can't cause a merge of allocating requests, the file will
fragment on the file system layer, with a potentially very small
fragment size (this depends on the requests the guest sent).
On Linux, fragmentation can be reduced by setting an extent size hint
when creating the file (at least on XFS, it can't be set any more after
the first extent has been allocated), basically giving raw files a
"cluster size" for allocation.
This adds a create option to set the extent size hint, and changes the
default from not setting a hint to setting it to 1 MB. The main reason
why qcow2 defaults to smaller cluster sizes is that COW becomes more
expensive, which is not an issue with raw files, so we can choose a
larger size. The tradeoff here is only potentially wasted disk space.
For qcow2 (or other image formats) over file-posix, the advantage should
even be greater because they grow sequentially without leaving holes, so
there won't be wasted space. Setting even larger extent size hints for
such images may make sense. This can be done with the new option, but
let's keep the default conservative for now.
The effect is very visible with a test that intentionally creates a
badly fragmented file with qemu-img bench (the time difference while
creating the file is already remarkable) and then looks at the number of
extents and the time a simple "qemu-img map" takes.
Without an extent size hint:
$ ./qemu-img create -f raw -o extent_size_hint=0 ~/tmp/test.raw 10G
Formatting '/home/kwolf/tmp/test.raw', fmt=raw size=10737418240 extent_size_hint=0
$ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 -S 8192 -o 0
Sending 1000000 write requests, 4096 bytes each, 64 in parallel (starting at offset 0, step size 8192)
Run completed in 25.848 seconds.
$ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 -S 8192 -o 4096
Sending 1000000 write requests, 4096 bytes each, 64 in parallel (starting at offset 4096, step size 8192)
Run completed in 19.616 seconds.
$ filefrag ~/tmp/test.raw
/home/kwolf/tmp/test.raw: 2000000 extents found
$ time ./qemu-img map ~/tmp/test.raw
Offset Length Mapped to File
0 0x1e8480000 0 /home/kwolf/tmp/test.raw
real 0m1,279s
user 0m0,043s
sys 0m1,226s
With the new default extent size hint of 1 MB:
$ ./qemu-img create -f raw -o extent_size_hint=1M ~/tmp/test.raw 10G
Formatting '/home/kwolf/tmp/test.raw', fmt=raw size=10737418240 extent_size_hint=1048576
$ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 -S 8192 -o 0
Sending 1000000 write requests, 4096 bytes each, 64 in parallel (starting at offset 0, step size 8192)
Run completed in 11.833 seconds.
$ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 1000000 -S 8192 -o 4096
Sending 1000000 write requests, 4096 bytes each, 64 in parallel (starting at offset 4096, step size 8192)
Run completed in 10.155 seconds.
$ filefrag ~/tmp/test.raw
/home/kwolf/tmp/test.raw: 178 extents found
$ time ./qemu-img map ~/tmp/test.raw
Offset Length Mapped to File
0 0x1e8480000 0 /home/kwolf/tmp/test.raw
real 0m0,061s
user 0m0,040s
sys 0m0,014s
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200707142329.48303-1-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
114 lines
3.2 KiB
Bash
Executable file
114 lines
3.2 KiB
Bash
Executable file
#!/usr/bin/env bash
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#
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# Test creating raw image preallocation mode
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#
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# Copyright (C) 2017 Nir Soffer <nirsof@gmail.com>
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#
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 2 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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#
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# creator
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owner=nirsof@gmail.com
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seq=`basename $0`
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echo "QA output created by $seq"
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status=1 # failure is the default!
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_cleanup()
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{
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_cleanup_test_img
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rm -f "$TEST_DIR/empty"
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}
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trap "_cleanup; exit \$status" 0 1 2 3 15
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# Some file systems sometimes allocate extra blocks independently of
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# the file size. This function hides the resulting difference in the
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# stat -c '%b' output.
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# Parameter 1: Number of blocks an empty file occupies
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# Parameter 2: Minimal number of blocks in an image
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# Parameter 3: Image size in bytes
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_filter_blocks()
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{
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extra_blocks=$1
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min_blocks=$2
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img_size=$3
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sed -e "s/blocks=$min_blocks\\(\$\\|[^0-9]\\)/min allocation/" \
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-e "s/blocks=$((extra_blocks + img_size / 512))\\(\$\\|[^0-9]\\)/max allocation/"
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}
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# Resize image using block_resize.
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# Parameter 1: image path
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# Parameter 2: new size
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_block_resize()
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{
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local path=$1
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local size=$2
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$QEMU -qmp stdio -nographic -nodefaults \
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-blockdev file,node-name=file,filename=$path,cache.direct=on \
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<<EOF
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{'execute': 'qmp_capabilities'}
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{'execute': 'block_resize', 'arguments': {'node-name': 'file', 'size': $size}}
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{'execute': 'quit'}
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EOF
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}
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# get standard environment, filters and checks
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. ./common.rc
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. ./common.filter
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_supported_fmt raw
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_supported_proto file
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_supported_os Linux
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_default_cache_mode none
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_supported_cache_modes none directsync
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size=$((1 * 1024 * 1024))
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touch "$TEST_DIR/empty"
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extra_blocks=$(stat -c '%b' "$TEST_DIR/empty")
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# We always write the first byte; check how many blocks this filesystem
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# allocates to match empty image alloation.
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printf "\0" > "$TEST_DIR/empty"
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min_blocks=$(stat -c '%b' "$TEST_DIR/empty")
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echo
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echo "== creating image with default preallocation =="
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_make_test_img -o extent_size_hint=0 $size | _filter_imgfmt
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stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $min_blocks $size
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for mode in off full falloc; do
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echo
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echo "== creating image with preallocation $mode =="
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_make_test_img -o preallocation=$mode,extent_size_hint=0 $size | _filter_imgfmt
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stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $min_blocks $size
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done
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for new_size in 4096 1048576; do
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echo
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echo "== resize empty image with block_resize =="
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_make_test_img -o extent_size_hint=0 0 | _filter_imgfmt
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_block_resize $TEST_IMG $new_size >/dev/null
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stat -c "size=%s, blocks=%b" $TEST_IMG | _filter_blocks $extra_blocks $min_blocks $new_size
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done
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# success, all done
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echo "*** done"
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rm -f $seq.full
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status=0
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