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

[Paolo Bonzini <pbonzini@redhat.com>]

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On 24/03/2016 16:25, Eric Blake wrote:
>> However, let's make these bits, so that
>>
>> NBD_STATE_ALLOCATED (0x1), LBA extent is present on the block device
>> NBD_STATE_ZERO (0x2), LBA extent will read as zeroes
> 
> Should we flip the sense and call this NBD_STATE_UNALLOCATED (0 means
> allocated, 1 means not present), so that an overall status of 0 is a
> safe default?

Double negations are evil (and don't work the same in all languages), so
I think it's a worse option.

>>     An implementation that doesn't track the "dirtiness" state of blocks
>>     MUST either fail this command with EINVAL, or mark all blocks as
>>     dirty in the descriptor that it returns.
> 
> Is it feasible to return zero/allocated/dirty status all at the same
> time, or do we want to strictly require two different modes of
> operation?

I think we should differentiate them, because it makes sense to support
only one.

In particular, while it is more or less obvious that (in my proposal
above) a trivial implementation must return NBD_STATE_ALLOCATED, it is
quite weird to require a trivial implementation to return
NBD_STATE_ALLOCATED|NBD_STATE_DIRTY.

Paolo

> That is, if we are returning zero and allocated as two bits,
> can we also return a third bit for dirty/clean?  Should we flip the
> sense of the bit, where 0 means dirty and 1 means clean, again so that a
> server can always return a status of 0 as the safe default?
> 


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