From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45961) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dsA9b-0001tt-9i for qemu-devel@nongnu.org; Wed, 13 Sep 2017 12:04:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dsA9a-0006KD-Ap for qemu-devel@nongnu.org; Wed, 13 Sep 2017 12:04:43 -0400 From: Eric Blake Date: Wed, 13 Sep 2017 11:03:24 -0500 Message-Id: <20170913160333.23622-15-eblake@redhat.com> In-Reply-To: <20170913160333.23622-1-eblake@redhat.com> References: <20170913160333.23622-1-eblake@redhat.com> Subject: [Qemu-devel] [PATCH v4 14/23] qemu-img: Speed up compare on pre-allocated larger file List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: famz@redhat.com, jsnow@redhat.com, kwolf@redhat.com, qemu-block@nongnu.org, Max Reitz 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 --- 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); -- 2.13.5