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kbase: incrementalbackupinternals: Clarify language in snapshots section

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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>
This commit is contained in:
Peter Krempa 2020-06-24 11:03:39 +02:00
parent 19061038a7
commit de56070c9d
1 changed files with 10 additions and 9 deletions
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19
docs/kbase/incrementalbackupinternals.rst
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@ -109,17 +109,18 @@ as ``base image``.
The topmost overlay is the image which is being written to by the VM and is also
described as the ``active`` layer or image.
Handling of bitmaps
-------------------
Handling of bitmaps during snapshots
------------------------------------
Creating an external snapshot involves adding a new layer to the backing chain
on top of the previous chain. In this step there are no new bitmaps created by
default, which would mean that backups become impossible after this step.
Creating an external snapshot involves adding a overlay on top of the previously
active image. Libvirt requires that all ``block-dirty-bitmaps`` which correspond
to the checkpoint must be created in the new overlay before any write from the
guest reaches the overlay to continue tracking which blocks are dirtied.
To prevent this from happening we need to re-create the active bitmaps in the
new top/active layer of the backing chain which allows us to continue tracking
the changes with same granularity as before and also allows libvirt to stitch
together all the corresponding bitmaps to do a backup across snapshots.
Since there are no new bitmaps created by ``qemu`` or ``qemu-img`` by default
when creating an overlay, we need to re-create the appropriate bitmaps
(see below) in the new overlay based on the previously active bitmaps in the
active image. The new bitmaps are created with the same granularity.
After taking a snapshot of the ``vda`` disk from the example above placed into
``vda-2.qcow2`` the following topology will be created: