Merge "Add warning about qcow2 version 2 format images" into stable/xena
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---
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issues:
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- |
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**NFS-based backend drivers and qcow2 version 2 images**
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Under some circumstances, NFS-based backend drivers will store a volume
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as a qcow2 image. Thus cinder allows for the possibility that an operator
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may choose to manage a storage object in an NFS-based backend that is a
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qcow2 image.
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Version 3 of the qcow2 format has been the default for qcow2 creation in
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qemu-img since QEMU-1.7 (December 2013), and operating system vendors are
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discussing discontinuing (or limiting) support of the version 2 format in
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upcoming releases.
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Thus, we recommend that operators who want to manage a storage object in
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an NFS-based storage backend as a cinder volume should not do this with a
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qcow2 image that is in the version 2 format, but should change it to the
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qcow2-v3 format first.
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.. note::
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The format version of a qcow2 can be determined by looking for the
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``compat`` field in the output of the ``qemu-img info`` command.
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A version 2 format image will report ``compat=0.10``, whereas a
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qcow2 in version 3 format will report ``compat=1.1``.
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