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?
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