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Images are stored as follows:
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/var/lib/uvtool/libvirt/images is a used as a libvirt volume pool called
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"uvtool". It is accessed through the libvirt API only.
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/var/lib/uvtool/libvirt/metadata contains files whose names correspond to
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the names defined in the "uvtool" libvirt volume pool.
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Metadata file semantics are as follows:
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If a metadata file does not exist, and the corresponding volume does not exist,
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then nothing is known about the volume (that doesn't exist).
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If a metadata file exists and the corresponding volume also exists, then the
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volume is known about and can be used. The metadata file is a JSON-formatted
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file containing the simplestreams key/value pairs associated with the image.
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If a metadata file does not exist, but the corresponding volume does exist,
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then the image is gone from the uvtool-simplestreams-libvirt view and cannot be
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used to create an instance. However, the image may still be in use by existing
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instances. It will be cleaned up on the next "uvtool-simplestreams-libvirt
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sync" command but only if it the volume is no longer in use. This is a common
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case: official "release" images go out of date, since updated images with new
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updated packages and kernels are continually produced. So the sync logic will
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drop images for use with new instances, but old instances will continue to use
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them as their copy-on-write backing volumes.
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If a metadata file exists, but the corresponding volume does not exist, then
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this is an error. "uvtool-simplestreams-libvirt sync" will erase the spurious
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metadata file on its next run.
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In all cases, a volume is treated as "in use" if an instance exists that uses
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it: either directly or through another volume as a backing store. These cases
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correspond to specific XPath searches in the libvirt domain and volume XML
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object definitions. Any other cases that should be covered need to be
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specifically added to the volume "in use" detection logic.
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