Re: [PATCH 4/4] kbase: Add document outlining backing chain XML config and troubleshooting

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On Tue, Dec 17, 2019 at 13:17:22 -0600, Eric Blake wrote:
> On 12/17/19 12:35 PM, Peter Krempa wrote:
> > Signed-off-by: Peter Krempa <pkrempa@xxxxxxxxxx>
> > ---

[...]

> > +If the ``backing file format:`` field is missing above the format was not
> > +specified properly. The image can be fixed by the following command:
> > +
> > +::
> > +
> > + qemu-img rebase -f $IMAGE_FORMAT -F $BACKING_IMAGE_FORMAT -b $BACKING_IMAGE_PATH $IMAGE_PATH
> 
> Adding -u can make this operation faster (blindly update the image metadata,
> rather than actually crawling the entire image to perform a data operation
> in the process).  But I'm not sure whether we want to document that here.

I thought qemu would skip the data verification if the backing image is
not being changed. I'm not sure about the -u here though.

> > +It is important to fill out ``$BACKING_IMAGE_FORMAT`` and ``$IMAGE_FORMAT``
> > +properly. ``$BACKING_IMAGE_PATH`` should be specified as a full absolute path.
> > +If relative referencing of the backing image is desired, the path must be
> > +relative to the location of image described by ``$IMAGE_PATH``.
> > +
> > +Missing images reported after after moving disk images into a different path
> > +----------------------------------------------------------------------------
> > +
> > +The path to the backing image which is recorded in the image metadata often
> > +contains a full path to the backing image. This is the default libvirt-created
> > +image configuration. When such images are moved to a different location the
> > +top image will no longer point to the correct image.
> > +
> > +To fix such issue you can either fully specify the image chain in the domain XML
> > +as pointed out above or the following ``qemu-img`` command can be used:
> > +
> > +::
> > +
> > + qemu-img rebase -u -f $IMAGE_FORMAT -F $BACKING_IMAGE_FORMAT -b $BACKING_IMAGE_PATH $IMAGE_PATH
> > +
> 
> Odd to mention -u here but not above.  Should we be consistent between the
> two examples?

You must use it here since qemu-img will fail to locate the old backing
file as the example is fixing a image chain using absolute paths moved
to a different location.

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