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From 7a2aff323f83e6f98f81a2cbe59005e34a094f93 Mon Sep 17 00:00:00 2001
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From: Maxim Levitsky <mlevitsk@redhat.com>
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Date: Wed, 5 Jun 2019 13:57:04 +0200
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Subject: [PATCH 16/23] qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing
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RH-Author: Maxim Levitsky <mlevitsk@redhat.com>
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Message-id: <20190605135705.24526-9-mlevitsk@redhat.com>
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Patchwork-id: 88558
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O-Subject: [RHEL-7.7 qemu-kvm-rhev PATCH 8/9] qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing
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Bugzilla: 1648622
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: John Snow <jsnow@redhat.com>
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From: Kevin Wolf <kwolf@redhat.com>
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If qemu-img convert sees that the target image isn't zero-initialised
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yet, it tries to do an efficient zero write for the whole image first
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to save the overhead of repeated explicit zero writes during the
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conversion. Obviously, this provides only an advantage if the
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pre-zeroing is actually efficient. Otherwise, we can end up writing
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zeroes slowly while zeroing out the whole image, and then overwrite the
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same blocks again with real data, potentially doubling the written data.
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Pass BDRV_REQ_NO_FALLBACK to blk_make_zero() to avoid this case. If we
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can't efficiently zero out, we'll instead write explicit zeroes only if
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there is no data to be written to a block.
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Acked-by: Eric Blake <eblake@redhat.com>
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Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1648622
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Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
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(Cherry-picked from c9fdcf202f19fc2acdcb1ee0522ff5d61bf8c906, no conflicts)
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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qemu-img.c | 2 +-
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1 file changed, 1 insertion(+), 1 deletion(-)
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diff --git a/qemu-img.c b/qemu-img.c
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index 5be2abf..d588183 100644
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--- a/qemu-img.c
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+++ b/qemu-img.c
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@@ -1916,7 +1916,7 @@ static int convert_do_copy(ImgConvertState *s)
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if (!s->has_zero_init && !s->target_has_backing &&
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bdrv_can_write_zeroes_with_unmap(blk_bs(s->target)))
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{
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- ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP);
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+ ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK);
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if (ret == 0) {
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s->has_zero_init = true;
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}
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--
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1.8.3.1
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