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From eea45924903f03dc6d8f20576be0a4a84d5acce4 Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Wed, 10 Feb 2021 10:16:11 -0500
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Subject: [PATCH 4/5] qemu-img convert: Don't pre-zero images
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <20210210101611.137928-2-kwolf@redhat.com>
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Patchwork-id: 101030
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O-Subject: [RHEL-8.4.0 qemu-kvm PATCH 1/1] qemu-img convert: Don't pre-zero images
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Bugzilla: 1855250
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RH-Acked-by: Stefano Garzarella <sgarzare@redhat.com>
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RH-Acked-by: Laszlo Ersek <lersek@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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Since commit 5a37b60a61c, qemu-img create will pre-zero the target image
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if it isn't already zero-initialised (most importantly, for host block
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devices, but also iscsi etc.), so that writing explicit zeros wouldn't
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be necessary later.
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This could speed up the operation significantly, in particular when the
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source image file was only sparsely populated. However, it also means
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that some block are written twice: Once when pre-zeroing them, and then
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when they are overwritten with actual data. On a full image, the
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pre-zeroing is wasted work because everything will be overwritten.
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In practice, write_zeroes typically turns out faster than writing
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explicit zero buffers, but slow enough that first zeroing everything and
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then overwriting parts can be a significant net loss.
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Meanwhile, qemu-img convert was rewritten in 690c7301600 and zero blocks
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are now written to the target using bdrv_co_pwrite_zeroes() if the
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target could be pre-zeroed. This way we already make use of the faster
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write_zeroes operation, but avoid writing any blocks twice.
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Remove the pre-zeroing because these days this former optimisation has
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actually turned into a pessimisation in the common case.
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Reported-by: Nir Soffer <nsoffer@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Message-Id: <20200622151203.35624-1-kwolf@redhat.com>
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Tested-by: Nir Soffer <nsoffer@redhat.com>
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Reviewed-by: Eric Blake <eblake@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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(cherry picked from commit edafc70c0c8510862f2f213a3acf7067113bcd08)
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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qemu-img.c | 9 ---------
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1 file changed, 9 deletions(-)
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diff --git a/qemu-img.c b/qemu-img.c
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index a27ad70851..b10dc5129b 100644
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--- a/qemu-img.c
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+++ b/qemu-img.c
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@@ -2029,15 +2029,6 @@ static int convert_do_copy(ImgConvertState *s)
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s->has_zero_init = false;
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}
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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 | 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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-
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/* Allocate buffer for copied data. For compressed images, only one cluster
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* can be copied at a time. */
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if (s->compressed) {
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--
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2.27.0
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