On Wed, Jun 10, 2015 at 11:19:30AM +0300, 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:
@@ -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?
65535 seems practical.
+=== 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?
Yes, that is good.
+
+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.
You are right, it wasn't clear when I read this the first time. My
problem was the "offset into the bitmap" doesn't have any units. So
let's make this more explicit. Can you document how to go from a bit
number down to the offset?