Re: [Qemu-block] [PATCH v2 2/2] iotests: simple mirror test with kvm on 1G image

[Max Reitz]

On 30.11.18 15:51, Vladimir Sementsov-Ogievskiy wrote:
> 30.11.2018 17:10, Kevin Wolf wrote:
>> Am 30.11.2018 um 14:48 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>> 30.11.2018 16:13, Max Reitz wrote:
>>>> On 30.11.18 14:06, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 30.11.2018 15:30, Max Reitz wrote:
>>>>>> On 29.11.18 11:18, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>> This test is broken without previous commit fixing dead-lock in mirror.
>>>>>>>
>>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy<address@hidden>
>>>>>>> ---
>>>>>>>     tests/qemu-iotests/235     | 59 
>>>>>>> ++++++++++++++++++++++++++++++++++++++
>>>>>>>     tests/qemu-iotests/235.out |  1 +
>>>>>>>     tests/qemu-iotests/group   |  1 +
>>>>>>>     3 files changed, 61 insertions(+)
>>>>>>>     create mode 100755 tests/qemu-iotests/235
>>>>>>>     create mode 100644 tests/qemu-iotests/235.out
>>>>>> I'll get to the first patch in a second, but first a suggestion for this
>>>>>> patch: I think it's not so good to use 2 GB of space for a test (1 GB
>>>>>> for the source, 1 GB for the target).  So I tried my luck and found that
>>>>>> the test works, too, if you just use preallocation=metadata for the
>>>>>> source (instead of actually writing data) and blockdev-mirror'ing the
>>>>>> data to a throttled null-co device.
>>>>>
>>>>> Hmm, so parsing metadata is enough for qcow2 to yield on write, yes?
>>>>
>>>> Apparently so.  If you can confirm that applying those changes to the
>>>> test still make it work (i.e., fail before patch 1, pass afterwards),
>>>> then I think it is just as good.
>>>
>>> Ok, I've checked that your changes works for me.
>>>
>>> hm, but we write to null, so, yield on write comes from throttling, however,
>>> without preallocation=metadata, it don't work.., do you know, why we need
>>> preallocation to reproduce?
>>
>> If I should take a guess, probably because mirror only copies allocated
>> clusters?
>>
>> Kevin
>>
> 
> hm, so, what preallocation=metadata does? I thought, it preallocates tables, 
> but all qcow2 clusters would be unallocated..

No, it initializes all metadata structures (i.e., L2 tables) so they
point to data clusters; which is why you cannot use it with backing
files.  It's just that the data clusters are not written.

It might make sense to use preallocated zero clusters in qcow2 v3; but
then writing to the image would cause COWs again, so there wouldn't be
too much benefit over just not preallocating.

Max

signature.asc
Description: OpenPGP digital signature