Re: [PATCH v5 2/5] qcow2: Expose bitmaps' size during measure

[Eric Blake]

On 5/21/20 8:17 AM, Nir Soffer wrote:

> > > I'm ok with both options, can we have only one?
> >
> > That was the crux of the conversation after v3, where we were trying to
> > figure out what interface you actually needed.  I implemented both to
> > show the difference, but if you want only one, then my preference is to
> > delete the --bitmaps option and only expose the optional 'bitmaps size'
> > field in all cases where bitmaps can be copied.
>
> I'm fine with this approach - but the bitmaps optional field should be displayed
> even if there are no bitmaps in the source, so I can tell if tihs
> version of qemu-img
> supports measuring/copying bitmaps.

The optional field appears when both source and destination have 
bitmaps.  That is, the field is present in all cases in which 'qemu-img 
convert --bitmaps' will succeed, even when the field is 0 because there 
is nothing to copy.

> If measure reports bitmaps size we will create a large enough disk and
> copy the bitmaps,
> and if not we will have to drop the relevant backup history for this
> vm, since the copy
> will not include the bitmaps. The next backup for this vm will have to
> be a full backup.

If you are always using qcow2 v3 images for both source and destination, 
then the field will be present, even when it is 0.  It will only be 
absent when there is nothing to copy (either the source or destination 
format is not qcow2, or you used measure on a size rather than on a 
source image).

> > Here's the diff that would accomplish that:
>
> Diff does not eliminate the "bitmaps: 0" outputs, so it looks good.

Okay, I'll revise the patch to get rid of the extra option (that is, 
getting it back to what it was in v3, before we had the discussion that 
prompted the addition of --bitmaps for comparison).  (I also have to 
touch up the iotests here and in 5/5)

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org