13.09.2017 19:03, Eric Blake wrote:
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 <ebl...@redhat.com>

---
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);

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>

--
Best regards,
Vladimir

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