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qemu-img: align result of is_allocated_sectors

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We currently don't enforce that the sparse segments we detect during convert are
aligned. This leads to unnecessary and costly read-modify-write cycles either
internally in Qemu or in the background on the storage device as nearly all
modern filesystems or hardware have a 4k alignment internally.

This patch modifies is_allocated_sectors so that its *pnum result will always
end at an alignment boundary. This way all requests will end at an alignment
boundary. The start of all requests will also be aligned as long as the results
of get_block_status do not lead to an unaligned offset.

The number of RMW cycles when converting an example image [1] to a raw device that
has 4k sector size is about 4600 4k read requests to perform a total of about 15000
write requests. With this path the additional 4600 read requests are eliminated while
the number of total write requests stays constant.

[1] https://cloud-images.ubuntu.com/releases/16.04/release/ubuntu-16.04-server-cloudimg-amd64-disk1.vmdk

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Peter Lieven 2018-07-12 15:00:10 +02:00 • committed by Kevin Wolf
parent 740842c965
commit 8dcd3c9b91
2 changed files with 46 additions and 16 deletions
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18
tests/qemu-iotests/122.out
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@ -194,12 +194,12 @@ wrote 1024/1024 bytes at offset 17408
1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
convert -S 4k
[{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 1024, "length": 7168, "depth": 0, "zero": true, "data": false},
{ "start": 8192, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 9216, "length": 8192, "depth": 0, "zero": true, "data": false},
{ "start": 17408, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 18432, "length": 67090432, "depth": 0, "zero": true, "data": false}]
[{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 4096, "length": 4096, "depth": 0, "zero": true, "data": false},
{ "start": 8192, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 12288, "length": 4096, "depth": 0, "zero": true, "data": false},
{ "start": 16384, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 20480, "length": 67088384, "depth": 0, "zero": true, "data": false}]
convert -c -S 4k
[{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true},
@ -210,10 +210,8 @@ convert -c -S 4k
{ "start": 18432, "length": 67090432, "depth": 0, "zero": true, "data": false}]
convert -S 8k
[{ "start": 0, "length": 9216, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 9216, "length": 8192, "depth": 0, "zero": true, "data": false},
{ "start": 17408, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 18432, "length": 67090432, "depth": 0, "zero": true, "data": false}]
[{ "start": 0, "length": 24576, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 24576, "length": 67084288, "depth": 0, "zero": true, "data": false}]
convert -c -S 8k
[{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true},