From c9fdcf202f19fc2acdcb1ee0522ff5d61bf8c906 Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Fri, 22 Mar 2019 13:49:28 +0100 Subject: [PATCH] qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing If qemu-img convert sees that the target image isn't zero-initialised yet, it tries to do an efficient zero write for the whole image first to save the overhead of repeated explicit zero writes during the conversion. Obviously, this provides only an advantage if the pre-zeroing is actually efficient. Otherwise, we can end up writing zeroes slowly while zeroing out the whole image, and then overwrite the same blocks again with real data, potentially doubling the written data. Pass BDRV_REQ_NO_FALLBACK to blk_make_zero() to avoid this case. If we can't efficiently zero out, we'll instead write explicit zeroes only if there is no data to be written to a block. Signed-off-by: Kevin Wolf Acked-by: Eric Blake --- qemu-img.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/qemu-img.c b/qemu-img.c index 5fac840742..8ee63daeae 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -1932,7 +1932,7 @@ static int convert_do_copy(ImgConvertState *s) if (!s->has_zero_init && !s->target_has_backing && bdrv_can_write_zeroes_with_unmap(blk_bs(s->target))) { - ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP); + ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK); if (ret == 0) { s->has_zero_init = true; } -- 2.39.2