Re: [Qemu-devel] blkdebug and VMDK (iotests 033 failure on monolithicFlat)

[Max Reitz <mreitz@redhat.com>]

On 06.05.2015 16:38, Fam Zheng wrote:
> On Wed, 05/06 16:12, Max Reitz wrote:
>> On 06.05.2015 15:59, Fam Zheng wrote:
>>> On Wed, 05/06 15:45, Max Reitz wrote:
>>>> On 05.05.2015 12:44, Fam Zheng wrote:
>>>>> Max,
>>>>>
>>>>> Since you once fixed VMDK with the json descriptor filename, could you take a
>>>>> look at the error:
>>>>>
>>>>> $ ./check -vmdk -o "subformat=monolithicFlat" 033
>>>>>
>>>>> 033         - output mismatch (see 033.out.bad)
>>>>> --- /home/fam/qemu/tests/qemu-iotests/033.out   2015-05-05 10:52:50.524378312 +0800
>>>>> +++ 033.out.bad 2015-05-05 17:38:36.147369924 +0800
>>>>> @@ -2,54 +2,36 @@
>>>>>   Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
>>>>>   == preparing image ==
>>>>> -wrote 1024/1024 bytes at offset 512
>>>>> -1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>>>> -wrote 1536/1536 bytes at offset 131072
>>>>> -1.500 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>>>> -wrote 131072/131072 bytes at offset 1024
>>>>> -128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>>>>> +qemu-io: can't open device blkdebug::/tmp/qemu-iotests/t.vmdk: Cannot use relative extent paths with VMDK descriptor file 'json:{"image": {"driver": "file", "filename": "/tmp/qemu-iotests/t.vmdk"}, "driver": "blkdebug", "align": "512"}'
>>>>> +qemu-io: can't open device blkdebug::/tmp/qemu-iotests/t.vmdk: Cannot use relative extent paths with VMDK descriptor file 'json:{"image": {"driver": "file", "filename": "/tmp/qemu-iotests/t.vmdk"}, "driver": "blkdebug", "align": "512"}'
>>>>> +qemu-io: can't open device blkdebug::/tmp/qemu-iotests/t.vmdk: Cannot use relative extent paths with VMDK descriptor file 'json:{"image": {"driver": "file", "filename": "/tmp/qemu-iotests/t.vmdk"}, "driver": "blkdebug", "align": "512"}'
>>>>>
>>>>> ...
>>>>>
>>>>>
>>>>> I'm not sure what's the best fix here. It's possible to dig out
>>>>> bs->file->file->filename in this case, but I'm not sure what's the rule of this
>>>>> filename mystery.
>>>>>
>>>>> Fam
>>>> Hi Fam,
>>>>
>>>> The problem is that the align option is (consciously) set for blkdebug, so
>>>> the blkdebug driver cannot just convert it to a blkdebug::$foo filename, but
>>>> must create a JSON filename so that the align option is included.
>>>>
>>>> I seem to recall that someone (me? I don't know) once proposed a way for
>>>> block drivers not only to present a file name (which is json:{${baz}} in
>>>> this case), but also the name of the directory they are in, or better, a
>>>> template to which a relative path can be appended (in Unix syntax, that is,
>>>> with / as separators and .. and so on). The difference to the file name
>>>> would be that we can ignore all the options given to the block driver for
>>>> controlling its behavior, we just need a path name.
>>>>
>>>> I don't know why we didn't pursue that proposal, but it does seem to be
>>>> necessary here.
>>>>
>>>> I think we can integrate it into the existing bdrv_refresh_filename()
>>>> "infrastructure", so it shouldn't be too difficult. I can take a shot at it,
>>>> if you agree.
>>>>
>>> Sounds consistent with what we have now, but to be honest I don't have the
>>> background to agree or disagree. Could you explain (again) why does filename
>>> have to reflect its option?
>> Because in a certain way, having the align option or not results in a
>> different file (at least in a different behavior). While one could argue
>> that in this case, it does not make that much of a difference, one can
>> easily conceive scenarios in which it does (for instance, for blkdebug it
>> might be an inject-error option). The idea is that the filename reported for
>> a BDS should result in the same BDS being constructed when used, or at least
>> one which behaves in the same way.
> Who's making use of the json file name?

qemu-img info and block-info, for one. Other than that, it can be used 
as a backing file name (where you can only specify a filename, but no 
additional options).

Max

>> If you open a file named blkdebug::x.img
>> with -o inject-error.event=read_aio, then just reporting blkdebug::x.img
>> would not be correct, since opening a BDS using the filename blkdebug::x.img
>> alone would not result in a BDS behaving the same. Therefore, we have to
>> generate a JSON object as the filename which includes this option.
>>
>> So while we could solve the problem in this case by ignoring the "align"
>> option when constructing the filename for blkdebug, it would not work for
>> the general case.
>>
>