Re: [Qemu-devel] [PATCH] iotests: Test non-self-referential qcow2 refblocks

[Max Reitz <mreitz@redhat.com>]

On 2014-12-05 at 12:43, Eric Blake wrote:
> On 12/05/2014 09:53 AM, Max Reitz wrote:
>> It is easy to create only self-referential refblocks, but there are
>> cases where that is impossible. This adds a test for two of those cases
>> (combined in a single test case).
>>
>> Suggested-by: Eric Blake <eblake@redhat.com>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>> This patch depends on version 4 (or hopefully any later version) of my
>> "qcow2: Support refcount orders != 4" series.
>> ---
>>   tests/qemu-iotests/115     | 95 ++++++++++++++++++++++++++++++++++++++++++++++
>>   tests/qemu-iotests/115.out |  8 ++++
>>   tests/qemu-iotests/group   |  1 +
>>   3 files changed, 104 insertions(+)
>>   create mode 100755 tests/qemu-iotests/115
>>   create mode 100644 tests/qemu-iotests/115.out
>> +
>> +# One refblock can describe (with cluster_size=512 and refcount_bits=64)
>> +# 512/8 = 64 clusters, therefore the L1 table should cover 128 clusters, which
>> +# equals 128 * (512/8) = 8192 entries (actually, 8192 - 512/8 = 8129 would
> This math is slightly off; you really only need 127 consecutive clusters
> to guarantee that you have an aligned 64 clusters somewhere in the mix.
>   Also, 8192 - 512/8 = 8128 (you are missing the +1 L2 entry that is
> necessary to ensure the rollover to 128 clusters).  The real minimum of
> L2 entries to provoke the situation we are after is thus 127 * (512/8) -
> 512/8 + 1 = 8065...
>
>> +# suffice, but it does not really matter). 8192 L2 tables can in turn describe
> ...at any rate, your conclusion that it does not really matter is
> correct; and rounding up to the actual power of 2 is both easier to
> explain and more likely to happen in practice (I'm not going to go out
> of my way to create a 255.9M guest image, after all).
>
> So, whether or not you want to tweak the comment wording, the test
> itself is correct.

Once again, I'm super-fast with my responses.

You're right. However, since the wording is not "This is the minimum for 
getting a non-self-referential refblock" but more like "If we do this, 
we are sure to get a non-self-referential refblock", I'll just leave it 
as it is.

> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks!