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From: | Vladimir Sementsov-Ogievskiy |
Subject: | Re: [PATCH] qemu-{img,nbd}: Don't report zeroed cluster as a hole |
Date: | Fri, 11 Jun 2021 11:14:22 +0300 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 |
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, 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.
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. -- Best regards, Vladimir
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