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From 2256db09e8b86b58be3c86b1575d64d7a9f5d05c Mon Sep 17 00:00:00 2001
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Message-Id: <2256db09e8b86b58be3c86b1575d64d7a9f5d05c@dist-git>
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From: Peter Krempa <pkrempa@redhat.com>
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Date: Fri, 28 Feb 2020 10:24:44 +0100
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Subject: [PATCH] kbase: backing_chains: Clarify some aspects of image probing
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Signed-off-by: Peter Krempa <pkrempa@redhat.com>
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Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
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(cherry picked from commit d552b93448e253552c7e53a1240132c9763d2b24)
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https://bugzilla.redhat.com/show_bug.cgi?id=1798148
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Message-Id: <b44052c081a18bec62762ab2a7358abd0e914a3a.1582881363.git.pkrempa@redhat.com>
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Reviewed-by: Ján Tomko <jtomko@redhat.com>
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---
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docs/kbase/backing_chains.rst | 16 ++++++++++++++--
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1 file changed, 14 insertions(+), 2 deletions(-)
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diff --git a/docs/kbase/backing_chains.rst b/docs/kbase/backing_chains.rst
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index 3b3f0583e5..12ed6253ac 100644
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--- a/docs/kbase/backing_chains.rst
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+++ b/docs/kbase/backing_chains.rst
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@@ -46,14 +46,17 @@ system used on the host so that the hypervisor can access the files and possibly
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also directly to configure the hypervisor to use the appropriate images. Thus
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it's important to properly setup the formats and paths of the backing images.
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+Any externally created image should always use the -F switch of ``qemu-img``
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+to specify the format of the backing file to avoid probing.
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+
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Image detection caveats
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-----------------------
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Detection of the backing chain requires libvirt to read and understand the
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``backing file`` field recorded in the image metadata and also being able to
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recurse and read the backing file. Due to security implications libvirt
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-will not attempt to detect the format of the backing image if the image metadata
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-doesn't contain it.
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+will refuse to use backing images of any image whose format was not specified
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+explicitly in the XML or the overlay image itself.
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Libvirt also might lack support for a network disk storage technology and thus
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may be unable to visit and detect backing chains on such storage. This may
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@@ -104,6 +107,8 @@ Note that it's also possible to partially specify the chain in the XML but omit
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the terminating element. This will result into probing from the last specified
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``<backingStore>``
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+Any image specified explicitly will not be probed for backing file or format.
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+
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Manual image creation
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=====================
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@@ -113,6 +118,13 @@ them properly so that they work with libvirt as expected. The created disk
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images must contain the format of the backing image in the metadata. This
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means that the **-F** parameter of ``qemu-img`` must always be used.
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+::
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+
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+ qemu-img -f qcow2 -F qcow2 -b /path/to/backing /path/to/overlay
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+
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+Note that if '/path/to/backing' is relative the path is considered relative to
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+the location of '/path/to/overlay'.
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+
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Troubleshooting
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===============
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--
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2.25.1
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