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

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH] spec/qcow2: bitmaps: zero bitmap table offset
Date: Wed, 29 Jun 2016 19:34:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

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 <address@hidden>
>> ---
>> 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. :-)


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