Re: [Qemu-devel] [PATCH v3 2/6] add basic backup support to block driver

[Dietmar Maurer]

> > > I think you do.  You're wasting time reading unallocated clusters
> > > and checking that they are zero.  bdrv_is_allocated_above gives you
> > > the same information much more efficiently.
> >
> > I thought that just returns information if the data is allocated, or
> > if data is on backing file?
> >
> > Or is data guaranteed to be zero if bdrv_is_allocated_above() return 0?
> 
> Oh, I need to pass NULL for base to get that information?

I just posted v5 of the patch. But I get a slow down of 15% if I use 
bdrv_is_allocated_above.
(tested with empty qcow2 files.)

Please can you take a look at the code - maybe I am doing something wrong?