Re: [RFC PATCH v3 05/27] qcow2: Document the Extended L2 Entries feature

[Eric Blake]

On 2/20/20 9:54 AM, Max Reitz wrote:

> > +Subcluster Allocation Bitmap (for standard clusters):
> > +
> > +    Bit  0 -  31:   Allocation status (one bit per subcluster)
> > +
> > +                    1: the subcluster is allocated. In this case the
> > +                       host cluster offset field must contain a valid
> > +                       offset.
> > +                    0: the subcluster is not allocated. In this case
> > +                       read requests shall go to the backing file or
> > +                       return zeros if there is no backing file data.
> > +
> > +                    Bits are assigned starting from the most significant one.
> > +                    (i.e. bit x is used for subcluster 31 - x)
>
> I still prefer it the other way round, both personally (e.g. it’s the C
> ordering), and because other places in qcow2 use LSb for bit ordering
> (the refcount order).

Internal consistency with refcount order using LSb ordering is the 
strongest reason to flip things, and have bit x be subcluster x.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org