Re: [Qemu-devel] [PATCH for 2.6 2/3] block: Hide HBitmap in block dirty bitmap interface

[Fam Zheng]

On Mon, 11/23 16:34, John Snow wrote:
> Hmm, what's the idea, here?
> 
> This patch does a lot more than just hide hbitmap details from callers
> of block_dirty_bitmap functions.
> 
> So we're changing the backing hbitmap to always be one where g=0 and the
> number of physical bits directly is (now) the same as the number of
> 'virtual' bits, pre-patch. Then, to compensate, we handle the shift math
> to convert the bitmap granularity to sector size and vice-versa in the
> Block Dirty Bitmap layer instead of in the hbitmap layer.
> 
> What's the benefit? It looks like we just pull all the implementation
> details up from hbitmap and into BdrvDirtyBitmap, which I am not
> immediately convinced of as being a benefit.

It feels counter intuitive to me with hbitmap handling granularity, it makes it
more like a HGranularityBitmap rather than HBitmap, and is unnecessarily
complex to work on.

Now it's simplified in that only one BdrvDirtyBitmap needs to care about the
granularity, and which I think is a big benefit when we are going to extend the
dirty bitmap interface, for example to serialize and deserialize for
persistence.

Fam