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kbase: backing_chains: Clarify some aspects of image probing

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Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
This commit is contained in:
Peter Krempa 2020-02-25 13:39:42 +01:00
parent ae9e6c2a2b
commit d552b93448
1 changed files with 14 additions and 2 deletions
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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
also directly to configure the hypervisor to use the appropriate images. Thus also directly to configure the hypervisor to use the appropriate images. Thus
it's important to properly setup the formats and paths of the backing images. it's important to properly setup the formats and paths of the backing images.
Any externally created image should always use the -F switch of ``qemu-img``
to specify the format of the backing file to avoid probing.
Image detection caveats Image detection caveats
----------------------- -----------------------
Detection of the backing chain requires libvirt to read and understand the Detection of the backing chain requires libvirt to read and understand the
``backing file`` field recorded in the image metadata and also being able to ``backing file`` field recorded in the image metadata and also being able to
recurse and read the backing file. Due to security implications libvirt recurse and read the backing file. Due to security implications libvirt
will not attempt to detect the format of the backing image if the image metadata will refuse to use backing images of any image whose format was not specified
doesn't contain it. explicitly in the XML or the overlay image itself.
Libvirt also might lack support for a network disk storage technology and thus Libvirt also might lack support for a network disk storage technology and thus
may be unable to visit and detect backing chains on such storage. This may may be unable to visit and detect backing chains on such storage. This may
@ -104,6 +107,8 @@ Note that it's also possible to partially specify the chain in the XML but omit
the terminating element. This will result into probing from the last specified the terminating element. This will result into probing from the last specified
``<backingStore>`` ``<backingStore>``
Any image specified explicitly will not be probed for backing file or format.
Manual image creation Manual image creation
===================== =====================
@ -113,6 +118,13 @@ them properly so that they work with libvirt as expected. The created disk
images must contain the format of the backing image in the metadata. This images must contain the format of the backing image in the metadata. This
means that the **-F** parameter of ``qemu-img`` must always be used. means that the **-F** parameter of ``qemu-img`` must always be used.
::
qemu-img -f qcow2 -F qcow2 -b /path/to/backing /path/to/overlay
Note that if '/path/to/backing' is relative the path is considered relative to
the location of '/path/to/overlay'.
Troubleshooting Troubleshooting
=============== ===============