Re: [Qemu-devel] [PATCH v3 2/8] block: add basic backup support to block driver

[Dietmar Maurer]

> >> Hmm, right.  But do we need the bitmap at all?  We can just use
> >> > bdrv_is_allocated like bdrv_co_do_readv does.
> > If a write occur, we read and backup that cluster immediately (out of
> > order). So I am quite sure we need the bitmap.
> 
> This is the same as how copy-on-read happens during image streaming, and it
> doesn't use a bitmap.

But a read does not modify the content.