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kbase: incrementalbackupinternals: Clarify language in snapshots section
Emphasize what needs to happen and also that creating a snapshot doesn't create the appropriate bitmaps. Also mention that granularity is kept. Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>
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@ -109,17 +109,18 @@ as ``base image``.
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The topmost overlay is the image which is being written to by the VM and is also
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described as the ``active`` layer or image.
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Handling of bitmaps
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-------------------
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Handling of bitmaps during snapshots
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------------------------------------
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Creating an external snapshot involves adding a new layer to the backing chain
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on top of the previous chain. In this step there are no new bitmaps created by
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default, which would mean that backups become impossible after this step.
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Creating an external snapshot involves adding a overlay on top of the previously
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active image. Libvirt requires that all ``block-dirty-bitmaps`` which correspond
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to the checkpoint must be created in the new overlay before any write from the
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guest reaches the overlay to continue tracking which blocks are dirtied.
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To prevent this from happening we need to re-create the active bitmaps in the
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new top/active layer of the backing chain which allows us to continue tracking
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the changes with same granularity as before and also allows libvirt to stitch
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together all the corresponding bitmaps to do a backup across snapshots.
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Since there are no new bitmaps created by ``qemu`` or ``qemu-img`` by default
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when creating an overlay, we need to re-create the appropriate bitmaps
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(see below) in the new overlay based on the previously active bitmaps in the
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active image. The new bitmaps are created with the same granularity.
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After taking a snapshot of the ``vda`` disk from the example above placed into
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``vda-2.qcow2`` the following topology will be created:
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