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Date: Thu, 26 Nov 2015 13:13:24 +0300
From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Subject: Re: [Qemu-devel] [PATCH v2 1/1] parallels: add format spec
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To: Stefan Hajnoczi <stefanha@redhat.com>, "Denis V. Lunev" <den@openvz.org>
Cc: John Snow <jsnow@redhat.com>, qemu-devel@nongnu.org

On 26.11.2015 11:17, Stefan Hajnoczi wrote:
> On Mon, Nov 23, 2015 at 04:32:37PM +0300, Denis V. Lunev wrote:
>> From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>
>> This specifies Parallels image format as implemented in Parallels Cloud
>> Server 6.10
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Eric Blake <eblake@redhat.com>
>> CC: John Snow <jsnow@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>> v2: add license info
>>      switch to offsets from types in field descriptions
> Cool!  Thanks for publishing a specification.  This will help everyone
> understand the code.
>
>> +=== Dirty bitmaps feature ===
>> +
>> +This feature provides a way of storing dirty bitmaps in the image. The fields
>> +of its data area are:
>> +
>> +   0 -  7:    size
>> +              The bitmap size, should be equal to disk size in sectors.
>> +
>> +   8 - 23:    id
>> +              An identifier for backup consistency checking.
>> +
>> +  24 - 27:    granularity
>> +              Bitmap granularity, in sectors. I.e., the number of sectors
>> +              corresponding to one bit of the bitmap.
> Does this need to be a power of 2?

Good point

>
>> +  28 - 31:    l1_size
>> +              The number of entries in the L1 table of the bitmap.
>> +
>> +  variable:   l1 (64 * l1_size bytes)
>> +              L1 offset table (in bytes)
>> +
>> +A dirty bitmap is stored using a one-level structure for the mapping to host
>> +clusters - an L1 table.
>> +
>> +Given an offset into the bitmap, the offset in bytes into the image file can be
> What are the units of the offset into the bitmap?  Is it a bit number
> (i.e. sector number / granularity)?

No, here bitmap is considered as raw data, stored using L1, so offset is 
in bytes. Bytes of the bitmap itself, as raw binary data.

-- 
Best regards,
Vladimir
* now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.