11.06.2021 14:21, Kevin Wolf wrote:
Am 11.06.2021 um 10:14 hat Vladimir Sementsov-Ogievskiy geschrieben:
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.
Or how about exposing BDRV_BLOCK_ALLOCATED as 'allocated': 'bool'? Which
I think is what the conclusion was already for NBD, so doing the same in
'qemu-img map' would be consistent.
"allocated" is historically ambiguous: we never know exactly does it mean "occupy space on
disk" or "data (or zeroes) taken from this qcow2 image, not from backing".
Eric recently sent related patch to libnbd:
[libnbd PATCH] info: Avoid ambiguous 'allocated' terminology in mapping
This is, of course, almost the same as 'absolutely-completely-absent',
just without the negating the flag.
Kevin
--
Best regards,
Vladimir