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

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(a)redhat.com&gt; > --- > docs/kbase/incrementalbackupinternals.rst | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) Reviewed-by: Eric Blake <eblake(a)redhat.com&gt; > > 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