[PATCH 1/6] kbase: incrementalbackupinternals: Add snapshot terminology

Eric Blake eblake at redhat.com
Wed Jun 24 14:23:00 UTC 2020


On 6/24/20 9:07 AM, Peter Krempa wrote:
> Make it obvious what's meant by 'overlay' and 'backing image' for sake
> of extension of the document.
> 
> Signed-off-by: Peter Krempa <pkrempa at redhat.com>
> ---
>   docs/kbase/incrementalbackupinternals.rst | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)

Reviewed-by: Eric Blake <eblake at redhat.com>

> 
> diff --git a/docs/kbase/incrementalbackupinternals.rst b/docs/kbase/incrementalbackupinternals.rst
> index 0c4b4f7486..eada0d2935 100644
> --- a/docs/kbase/incrementalbackupinternals.rst
> +++ b/docs/kbase/incrementalbackupinternals.rst
> @@ -94,6 +94,21 @@ so, even if we obviously can't guarantee that.
>   Integration with external snapshots
>   ===================================
> 
> +External snapshot terminology
> +-----------------------------
> +
> +External snapshots on a disk level consist of layered chains of disk images. An
> +image in the chain can have a ``backing image`` placed below. Any chunk in the
> +current image which was not written explicitly is transparent and if read the
> +data from the backing image is passed through. An image placed on top of the
> +current image is called ``overlay``.
> +
> +The bottommost backing image at the end of the chain is also usually described
> +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.

Maybe it's also worth a paragraph mentioning how diagramming the chains 
we typically use <- for 'Backed by', as in:

Base <- Top

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




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