Re: [Qemu-devel] [PATCH] spec/qcow2: bitmaps: zero bitmap table offset

[Max Reitz <mreitz@redhat.com>]

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On 29.06.2016 15:24, Vladimir Sementsov-Ogievskiy wrote:
> On 29.06.2016 15:22, Vladimir Sementsov-Ogievskiy wrote:
>> This allows effectively free in_use bitmap clusters including bitmap
>> table without loss of meaningful data.

I'm afraid I fail to understand this sentence.

>>
>> Now it is possible only to free end-point clusters and zero-out (not
>> free) bitmap table

OK, so the idea is not having to store a bitmap table if the bitmap is
fully zeroed?

>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---
>>
>> Hi all!
>>
>> Here is one small but significant addition to specification of bitmaps
>> in qcow2.
>>
>> Can we apply it just like this or I'll have to inroduce new
>> incompatible feature flag?

I think it should be fine without a flag.

>>
>> If there is existing implementation of the format, it may break image,
>> saved by
>> software, using extended spec. But is there are any implementations
>> except not
>> finished my one?
>>
>>
>>   docs/specs/qcow2.txt | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
>> index 80cdfd0..dd07a82 100644
>> --- a/docs/specs/qcow2.txt
>> +++ b/docs/specs/qcow2.txt
>> @@ -435,6 +435,8 @@ Structure of a bitmap directory entry:
>>                       Offset into the image file at which the bitmap
>> table
>>                       (described below) for the bitmap starts. Must be
>> aligned to
>>                       a cluster boundary.
>> +                    Zero value means that bitmap table is not
>> allocated and the
>> +                    bitmap should be considered as empty (all bits
>> are zero).
>>              8 - 11:    bitmap_table_size
>>                       Number of entries in the bitmap table of the
>> bitmap.
> 
> +               bitmap_table_size must be zero if bitmap_table_size is
> zero.

The second bitmap_table_size should probably be bitmap_table_offset.

The idea looks good to me, but I'd rather understand the commit message
before giving an R-b. :-)

Max


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