[PATCH 2/6] kbase: incrementalbackupinternals: Clarify language in snapshots section

Eric Blake eblake at redhat.com
Wed Jun 24 14:25:45 UTC 2020

On 6/24/20 9:07 AM, Peter Krempa wrote:
> Emphaisze 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 at redhat.com>
> ---
>   docs/kbase/incrementalbackupinternals.rst | 23 ++++++++++++-----------
>   1 file changed, 12 insertions(+), 11 deletions(-)
> diff --git a/docs/kbase/incrementalbackupinternals.rst b/docs/kbase/incrementalbackupinternals.rst
> index eada0d2935..dbef1e96f3 100644
> --- a/docs/kbase/incrementalbackupinternals.rst
> +++ b/docs/kbase/incrementalbackupinternals.rst
> @@ -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
> --------------------
> -
> -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.
> -
> -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.
> +Handling of bitmaps during snapshots
> +------------------------------------
> +
> +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.
> +
> +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 (see below.)
> +bitmaps in the new overlay based on the previously active bitmaps in the active

s/(see below.) bitmaps/bitmaps (see below)/

> +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:

With the fixes,
Reviewed-by: Eric Blake <eblake at redhat.com>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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