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

[Kevin Wolf <kwolf@redhat.com>]

Am 10.06.2021 um 22:46 hat Eric Blake geschrieben:
> 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.

A negative value isn't any less unexpected than a missing key. I don't
think any existing tool would be able to handle it. Encoding different
meanings in a single value isn't very QAPI-like either. Usually strings
that are parsed are the problem, but negative integers really isn't that
much different. I don't really like this solution.

Leaving out the depth feels like a better suggestion to me.

But anyway, this seems to only happen at the end of the backing chain.
So if the backing chain consistents of n images, why not report 'depth':
n + 1? So, in the above example, you would get 1. I think this has the
best chances of tools actually working correctly with the new output,
even though it's still not unlikely to break something.

Kevin