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

[Kevin Wolf]

Am 24.03.2016 um 17:47 hat Wouter Verhelst geschrieben:
> 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.

I just thought that "trim" sounds more like an action than a status, and
while the reason for a hole to exist can be a previous TRIM command,
another option is that it's an area in an image that just has never been
written to. In that case "trim" would be a misnomer.

> 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.

Good point. That might be another reason for not calling the status
"trim".

Kevin

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