Re: [Qemu-devel] [PATCH 1/8] spec: add qcow2-dirty-bitmaps specification

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

On 10.06.2015 11:19, Vladimir Sementsov-Ogievskiy wrote:
> On 09.06.2015 20:03, Stefan Hajnoczi wrote:
>> On Mon, Jun 08, 2015 at 06:21:19PM +0300, Vladimir 
>> Sementsov-Ogievskiy wrote:
>>> From: Vladimir Sementsov-Ogievskiy <vsementsov@parallels.com>
>>>
>>> Persistent dirty bitmaps will be saved into qcow2 files. It may be used
>>> as 'internal' bitmaps (for qcow2 drives) or as 'external' bitmaps for
>>> other drives (there may be qcow2 file with zero disk size but with
>>> several dirty bitmaps for other drives).
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   docs/specs/qcow2.txt | 66 
>>> ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>   1 file changed, 66 insertions(+)
>>>
>>> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
>>> index 121dfc8..0fffba2 100644
>>> --- a/docs/specs/qcow2.txt
>>> +++ b/docs/specs/qcow2.txt
>>> @@ -123,6 +123,7 @@ be stored. Each extension has a structure like 
>>> the following:
>>>                           0x00000000 - End of the header extension area
>>>                           0xE2792ACA - Backing file format name
>>>                           0x6803f857 - Feature name table
>>> +                        0x23852875 - Dirty bitmaps
>>>                           other      - Unknown header extension, can 
>>> be safely
>>>                                        ignored
>>>   @@ -166,6 +167,19 @@ the header extension data. Each entry look 
>>> like this:
>>>                       terminated if it has full length)
>>>     +== Dirty bitmaps ==
>>> +
>>> +Dirty bitmaps is an optional header extension. It provides a 
>>> possibility of
>>> +storing dirty bitmaps in qcow2 image. The fields are:
>>> +
>>> +          0 -  3:  nb_dirty_bitmaps
>>> +                   Number of dirty bitmaps contained in the image
>> Is there a maximum?
> hmm. any proposals for this?
>>
>>> +
>>> +          4 - 11:  dirty_bitmaps_offset
>>> +                   Offset into the image file at which the dirty 
>>> bitmaps table
>>> +                   starts. Must be aligned to a cluster boundary.
>> The autoclear feature bit is undocumented.
>>
>>>   == Host cluster management ==
>>>     qcow2 manages the allocation of host clusters by maintaining a 
>>> reference count
>>> @@ -360,3 +374,55 @@ Snapshot table entry:
>>>             variable:   Padding to round up the snapshot table entry 
>>> size to the
>>>                       next multiple of 8.
>>> +
>>> +
>>> +== Dirty bitmaps ==
>>> +
>>> +The feature supports storing several dirty bitmaps in the qcow2 file.
>>> +
>>> +=== Cluster mapping ===
>>> +
>>> +Dirty bitmaps are stored using a ONE-level structure for the 
>>> mapping of
>>> +bitmaps to host clusters. There is only an L1 table.
>>> +
>>> +The L1 table has a variable size (stored in the Bitmap table entry) 
>>> and may
>>> +use multiple clusters, however it must be contiguous in the image 
>>> file.
>> The use of "L1 table" could be confusing.  The refcount metadata uses
>> "refcount table" and "refcount block" to describe a one-level table.
> I agree. Hmm.. dirty bitmaps table? ok?
oh, no, bad idea. dirty bitmaps table is other thing))
>>
>>> +
>>> +Given an offset into the bitmap, the offset into the image file can be
>>> +obtained as follows:
>>> +
>>> +    offset = l1_table[offset / cluster_size] + (offset % cluster_size)
>> It might help to add granularity to this formula.
>>
>> Instead of "offset", "bit_number" or "bitnr" might be clearer since
>> "offset" means something different in other parts of the document.
> Hmm. In my opinion, the bitmap here is stored as raw data. And 
> granularity is an additional parameter (for deserializing this data). 
> So, it is an offset in bytes for this data. The format is not for 
> accessing bitmap bits, it's only for loading the whole bitmap one time.
>>
>>> +
>>> +L1 table entry:
>>> +
>>> +    Bit  0 -  61:   Standard cluster descriptor
>>> +
>>> +        62 -  63:   Reserved
>> Do you really want to use the standard cluster descriptor with it's zero
>> bit?
>>
>> Since bitmaps don't honor backing files there doesn't seem much point in
>> using the zero bit, things are simpler if just bits 9-55 are contain the
>> host cluster offset and 0 means the cluster is unallocated.
>>
>> By honoring the zero bit there are three states:
>> 1. Zero bit set, read zeroes
>> 2. Zero bit not set, host cluster offset != 0, bits valid
>> 3. Zero bit not set, host cluster offset == 0, unallocated
>>
>> State 1 is not useful.
>>
>>> +=== Bitmap table ===
>>> +
>>> +A directory of all bitmaps is stored in the bitmap table, a 
>>> contiguous area in
>>> +the image file, whose starting offset and length are given by the 
>>> header fields
>>> +dirty_bitmaps_offset and nb_dirty_bitmaps. The entries of the 
>>> bitmap table have
>>> +variable length, depending on the length of name and extra data.
>>> +
>>> +Bitmap table entry:
>>> +
>>> +    Byte 0 -  7:    Offset into the image file at which the L1 
>>> table for the
>>> +                    bitmap starts. Must be aligned to a cluster 
>>> boundary.
>>> +
>>> +         8 - 11:    Number of entries in the L1 table of the bitmap
>>> +
>>> +        12 - 15:    Bitmap granularity in bytes
>>> +
>>> +        16 - 23:    Bitmap size in sectors
>>> +
>>> +        24 - 25:    Size of the bitmap name
>>> +
>>> +        variable:   The name of the bitmap (not null terminated)
>>> +
>>> +        variable:   Padding to round up the bitmap table entry size 
>>> to the
>>> +                    next multiple of 8.
>>> +
>>> +The fields "size", "granularity" and "name" are corresponding with 
>>> the fields
>>> +in struct BdrvDirtyBitmap.
>> Referring to the internals of a C struct in QEMU is not appropriate for
>> a file format specification.  Please document the fields fully including
>> their constraints, minimums, maximums, etc.
>
>


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