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
