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

[Nir Soffer]

On Thu, Jun 10, 2021 at 9:35 PM Eric Blake <eblake@redhat.com> wrote:
>
> On Tue, Jun 08, 2021 at 07:38:10PM +0300, Nir Soffer wrote:
> > The example I provided was not detailed enough, what we actually do is:
> >
> >     qemu-nbd .. 'json:{"driver": "qcow2", "backing": null, "file":
> > {"driver": "file", "filename": "top.qcow2"}}'
> >
> > So there is no backing chain and allocation depth is not relevant.
> > - Allocated areas should be reported with flags 0
> > - Zero areas which are not holes should be reported as NBD_STATE_ZERO
> > - Zero areas which are holes (not allocated in this image) should be
> > reported as NBD_STATE_HOLE
>
> Thinking about this a bit more, here's something I noticed:
>
> $ qemu-img map --output=json -f raw base.raw
> [{ "start": 0, "length": 196608, "depth": 0, "zero": false, "data": true, 
> "offset": 0},
> { "start": 196608, "length": 65536, "depth": 0, "zero": true, "data": false, 
> "offset": 196608}]
>
> which matches what I've said elsewhere in this thread: the entire
> image is reported as "depth":0 because the raw file is responsible for
> 100% of the content.
>
> 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}]