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

Peter Krempa pkrempa at redhat.com
Wed Jun 24 14:07:48 UTC 2020

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(+)

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.
 Handling of bitmaps


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