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@xxxxxxxxxx> --- docs/kbase/incrementalbackupinternals.rst | 15 +++++++++++++++ 1 file changed, 15 insertions(+)
Reviewed-by: Eric Blake <eblake@xxxxxxxxxx>
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
