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

[Pavel Borzenkov]

On Thu, Mar 24, 2016 at 09:41:29AM +0100, Wouter Verhelst wrote:
> On Thu, Mar 24, 2016 at 11:25:52AM +0300, Pavel Borzenkov wrote:
> > On Wed, Mar 23, 2016 at 07:14:54PM +0100, Kevin Wolf wrote:
> > > Am 23.03.2016 um 18:58 hat Wouter Verhelst geschrieben:
> > > > On Wed, Mar 23, 2016 at 05:16:02PM +0300, Denis V. Lunev wrote:
> > > > > +    the provisioning state of the device. The following provisionnig 
> > > > > states
> > > > > +    are defined for the command:
> > > > > +
> > > > > +      - `NBD_STATE_ALLOCATED` (0x0), LBA extent is present on the 
> > > > > block device;
> > > > > +      - `NBD_STATE_ZEROED` (0x1), LBA extent is present on the block 
> > > > > device
> > > > > +        and contains zeroes;
> > > > 
> > > > Presumably this should be "contains only zeroes"?
> > > > 
> > > > Also, this may end up being a fairly expensive call for the server to
> > > > process. Is it really useful?
> > > 
> > > I think we need to make clear that this is meant as an optimisation and
> > > it's always a valid option for a server to return NBD_STATE_ALLOCATED
> > > even if the contents is zeroed.
> > > 
> > > It is definitely useful if the server has a means to efficiently find
> > > out the allocation status (e.g. SEEK_HOLE). In that case the client may
> > > be able to avoid reading the block and sending it over the network, or
> > > when making a copy, it could use it to keep the target file sparse. If
> > > the client can't take advantage, we didn't have much overhead, so it's
> > > fine.
> > 
> > Yes, that was the idea. I'll add a note that the server may return
> > NBD_STATE_ALLOCATED instead of NBD_STATE_ZEROED if it has not means to
> > efficiently differentiate allocated blocks with zeroes from allocated
> > blocks with non-zeroed content.
> 
> Okay, that alleviates my concerns.
> 
> In that case it might be useful if the server could say something along
> the lines of "I know it's allocated, but I didn't check whether there's
> anything non-zero in there"? The client can then decide to do nothing
> with that information; but the more useful information is sent along,
> the better...

Doesn't allocated state mean exactly this? E.g. it is allocated and I
have no idea what the content is.

> 
> -- 
> < 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