Re: [Qemu-devel] [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension

[Paolo Bonzini]

On 24/03/2016 13:17, Alex Bligh wrote:
>>> >> * unallocated
>>> >> * zero
>>> >> * non-zero
>>> >> 
>>> >> So the possible replies are a bitfield of those, with a '1' if it 'might'
>>> >> be in that state, i.e.
>>> >> 
>>> >> 111 = no idea
>>> >> 110 = might be zero or unallocated, but isn't zero
>>> >> 011 = I know it's allocated, but I don't know whether it is zero or not
>> > 
>> > How do you represent "definitely unallocated?"
> 100 is definitely allocated. The first '1' says it 'might' be in allocated 
> state,
> but as we know it's NOT in any of the other states (next two zeroes), by a
> process of elimination, it's definitely unallocated. Similarly 010 and 001
> are the two other 'definite' states.

An unallocated block can still be definitely zero.

Paolo