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

[Stefan Hajnoczi]

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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