Re: [PATCH] qemu-{img,nbd}: Don't report zeroed cluster as a hole

[Eric Blake]

On Thu, Jun 10, 2021 at 11:09:05PM +0300, Nir Soffer wrote:
> > But:
> >
> > $ qemu-img map --output=json -f qcow2 
> > json:'{"driver":"qcow2","backing":null, \
> >   "file":{"driver":"file","filename":"top.qcow2"}}'
> > [{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false},
> > { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, 
> > "offset": 327680},
> > { "start": 131072, "length": 131072, "depth": 0, "zero": true, "data": 
> > false}]
> >
> > also reports the entire file at "depth":0, which is misleading, since
> > we have just been arguing from the qemu:allocation-depth perspective
> > (and also from bdrv_block_status) that the qcow2 image is NOT 100%
> > allocated (in the sense where allocation == data comes locally).
> > Perhaps it might be better if we tweaked the above qemu-img map to
> > produce:
> >
> > [{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
> > { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, 
> > "offset": 327680},
> > { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": 
> > false},
> > { "start": 196608, "length": 65536, "depth": -1, "zero": true, "data": 
> > false}]
> 
> It will be more consistent with "offset" to drop "depth" from output
> if we don't have it:
> 
>     [{ "start": 0, "length": 65536, "zero": true, "data": false},
>      { "start": 65536, "length": 65536, "depth": 0, "zero": false,
> "data": true, "offset": 327680},
>      { "start": 131072, "length": 65536, "depth": 0, "zero": true,
> "data": false},
>      { "start": 196608, "length": 65536, "zero": true, "data": false}]

Yes, that might work as well.  But we didn't previously document
depth to be optional.  Removing something from output risks breaking
more downstream tools that expect it to be non-optional, compared to
providing a new value.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org