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qemu-img: make is_allocated_sectors() more efficient

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Consider the case when the whole buffer is zero and end is unaligned.

If i <= tail, we return 1 and do one unaligned WRITE, RMW happens.

If i > tail, we do on aligned WRITE_ZERO (or skip if target is zeroed)
and again one unaligned WRITE, RMW happens.

Let's do better: don't fragment the whole-zero buffer and report it as
ZERO: in case of zeroed target we just do nothing and avoid RMW. If
target is not zeroes, one unaligned WRITE_ZERO should not be much worse
than one unaligned WRITE.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211217164654.1184218-3-vsementsov@virtuozzo.com>
Tested-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Vladimir Sementsov-Ogievskiy 2021-12-17 17:46:54 +01:00 • committed by Kevin Wolf
parent 51cd8bddd6
commit 96054c76ff
2 changed files with 21 additions and 10 deletions
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8
tests/qemu-iotests/122.out
View file

@ -201,9 +201,7 @@ convert -S 4k
{ "start": 8192, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
{ "start": 12288, "length": 4096, "depth": 0, "present": false, "zero": true, "data": false},
{ "start": 16384, "length": 4096, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
{ "start": 20480, "length": 46080, "depth": 0, "present": false, "zero": true, "data": false},
{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}]
{ "start": 20480, "length": 67088384, "depth": 0, "present": false, "zero": true, "data": false}]
convert -c -S 4k
[{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true},
@ -215,9 +213,7 @@ convert -c -S 4k
convert -S 8k
[{ "start": 0, "length": 24576, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
{ "start": 24576, "length": 41984, "depth": 0, "present": false, "zero": true, "data": false},
{ "start": 66560, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true, "offset": OFFSET},
{ "start": 67584, "length": 67041280, "depth": 0, "present": false, "zero": true, "data": false}]
{ "start": 24576, "length": 67084288, "depth": 0, "present": false, "zero": true, "data": false}]
convert -c -S 8k
[{ "start": 0, "length": 1024, "depth": 0, "present": true, "zero": false, "data": true},