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 <address@hidden>
---
v3: new patch
---
qemu-img.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/qemu-img.c b/qemu-img.c
index f8423e9b3f..f5ab29d176 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1477,11 +1477,11 @@ 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);
if (ret < 0) {
ret = 3;
error_report("Sector allocation test failed for %s",
@@ -1489,11 +1489,11 @@ 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);