From 7a2aff323f83e6f98f81a2cbe59005e34a094f93 Mon Sep 17 00:00:00 2001
From: Maxim Levitsky <mlevitsk@redhat.com>
Date: Wed, 5 Jun 2019 13:57:04 +0200
Subject: [PATCH 16/23] qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing

RH-Author: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: <20190605135705.24526-9-mlevitsk@redhat.com>
Patchwork-id: 88558
O-Subject: [RHEL-7.7 qemu-kvm-rhev PATCH 8/9] qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing
Bugzilla: 1648622
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: John Snow <jsnow@redhat.com>

From: Kevin Wolf <kwolf@redhat.com>

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 <kwolf@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1648622
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

(Cherry-picked from c9fdcf202f19fc2acdcb1ee0522ff5d61bf8c906, no conflicts)

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 qemu-img.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index 5be2abf..d588183 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1916,7 +1916,7 @@ 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;
         }
-- 
1.8.3.1