ב-11 ביוני 2021, בשעה 11:14, Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> כתב/ה:
11.06.2021 11:09, Kevin Wolf wrote: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, sincewe 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 toproduce:[{ "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 outputif 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 documentdepth to be optional. Removing something from output risks breakingmore downstream tools that expect it to be non-optional, compared toproviding a new value.A negative value isn't any less unexpected than a missing key. I don'tthink any existing tool would be able to handle it. Encoding differentmeanings in a single value isn't very QAPI-like either. Usually stringsthat are parsed are the problem, but negative integers really isn't thatmuch 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 thebest chances of tools actually working correctly with the new output,even though it's still not unlikely to break something.
Did you consider just add a new field?
So, "depth" keeps its meaning "which level provides data".
And we add additional optional field like
absolutely-completely-absent: bool
Which is true if data is nowhere in the backing chain.
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Best regards,
Vladimir