On Thu, Mar 24, 2016 at 05:07:47PM +0100, Kevin Wolf wrote:
> Am 24.03.2016 um 17:04 hat Eric Blake geschrieben:
> > On 03/24/2016 09:53 AM, Wouter Verhelst wrote:
> > > On Thu, Mar 24, 2016 at 04:33:42PM +0100, Paolo Bonzini wrote:
> > >> 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.
> > > 
> > > I agree that a bit which says "unallocated" is confusing in that manner,
> > > but that just means we need a better name (one that doesn't contain
> > > "un-" or "not")
> > > 
> > > I like the idea of having zero be the "sensible" default, although I'm
> > > quite unable to come up with a better name myself.
> > 
> > NBD_STATE_TRIM, perhaps? (0 for present, 1 for trimmed or unallocated);
> > matches well that we have NBD_CMD_TRIM for requesting the creation of
> > such a state.
> 
> How about NBD_STATE_HOLE?

Both will work, although I like NBD_STATE_TRIM slightly better because
it indeed nicely references NBD_CMD_TRIM.

However, I also think it should then be made clear that issuing
NBD_CMD_TRIM doesn't *require* that GET_BLOCK returns NBD_STATE_TRIM for
that region if the backend storage format dosn't support that, to avoid
confusion later on.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12