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qemu-img: Speed up compare on pre-allocated larger file

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Compare the following images with all-zero contents:
$ truncate --size 1M A
$ qemu-img create -f qcow2 -o preallocation=off B 1G
$ qemu-img create -f qcow2 -o preallocation=metadata C 1G

On my machine, the difference is noticeable for pre-patch speeds,
with more than an order of magnitude in difference caused by the
choice of preallocation in the qcow2 file:

$ time ./qemu-img compare -f raw -F qcow2 A B
Warning: Image size mismatch!
Images are identical.

real	0m0.014s
user	0m0.007s
sys	0m0.007s

$ time ./qemu-img compare -f raw -F qcow2 A C
Warning: Image size mismatch!
Images are identical.

real	0m0.341s
user	0m0.144s
sys	0m0.188s

Why? Because bdrv_is_allocated() returns false for image B but
true for image C, throwing away the fact that both images know
via lseek(SEEK_HOLE) that the entire image still reads as zero.
From there, qemu-img ends up calling bdrv_pread() for every byte
of the tail, instead of quickly looking for the next allocation.
The solution: use block_status instead of is_allocated, giving:

$ time ./qemu-img compare -f raw -F qcow2 A C
Warning: Image size mismatch!
Images are identical.

real	0m0.014s
user	0m0.011s
sys	0m0.003s

which is on par with the speeds for no pre-allocation.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Eric Blake 2017-10-11 22:47:10 -05:00 committed by Kevin Wolf
parent 7daddc615e
commit 391cb1aaca
1 changed files with 4 additions and 4 deletions
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8
qemu-img.c
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@ -1480,11 +1480,11 @@ static int img_compare(int argc, char **argv)
while (sector_num < progress_base) {
int64_t count;
ret = bdrv_is_allocated_above(blk_bs(blk_over), NULL,
ret = bdrv_block_status_above(blk_bs(blk_over), NULL,
sector_num * BDRV_SECTOR_SIZE,
(progress_base - sector_num) *
BDRV_SECTOR_SIZE,
&count);
&count, NULL, NULL);
if (ret < 0) {
ret = 3;
error_report("Sector allocation test failed for %s",
@ -1492,11 +1492,11 @@ static int img_compare(int argc, char **argv)
goto out;
}
/* TODO relax this once bdrv_is_allocated_above does not enforce
/* TODO relax this once bdrv_block_status_above does not enforce
* sector alignment */
assert(QEMU_IS_ALIGNED(count, BDRV_SECTOR_SIZE));
nb_sectors = count >> BDRV_SECTOR_BITS;
if (ret) {
if (ret & BDRV_BLOCK_ALLOCATED && !(ret & BDRV_BLOCK_ZERO)) {
nb_sectors = MIN(nb_sectors, IO_BUF_SIZE >> BDRV_SECTOR_BITS);
ret = check_empty_sectors(blk_over, sector_num, nb_sectors,
filename_over, buf1, quiet);