[PATCH 1/6] kbase: incrementalbackupinternals: Add snapshot terminology
pkrempa at redhat.com
Thu Jun 25 09:57:58 UTC 2020
On Wed, Jun 24, 2020 at 09:23:00 -0500, Eric Blake wrote:
> 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
Good idea! I'll follow up with a patch to add the paragraph.
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