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

[Alex Bligh <alex@...872...>]

On 24 Mar 2016, at 12:32, Paolo Bonzini <pbonzini@...696...> wrote:

> 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.

Sorry, I should have been clearer on the states:

bits
210

1-- Unallocated, and hence reads as zero
-1- Allocated, and reads as zero
--1 Allocated, and reads as non-zero

So 100 means 'definitely unallocated, will read as zero'.

If you are saying that there is also a state called 'Unallocated, but reads
as non-zero', that could be handled by adding a fourth bit. Same idea. One
would presume this would only ever be set in conjunction with bit 2.

My point in general was to represent all the possible states and let the client
determine what it wants to do with the information.

-- 
Alex Bligh