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kbase: backing_chains: Clarify some aspects of image probing
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
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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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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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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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Image detection caveats
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Image detection caveats
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-----------------------
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-----------------------
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Detection of the backing chain requires libvirt to read and understand the
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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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``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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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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will refuse to use backing images of any image whose format was not specified
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doesn't contain it.
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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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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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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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the terminating element. This will result into probing from the last specified
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``<backingStore>``
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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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Manual image creation
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Manual image creation
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=====================
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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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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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means that the **-F** parameter of ``qemu-img`` must always be used.
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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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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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Troubleshooting
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Troubleshooting
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===============
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===============
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