On Wed, Feb 12, 2020 at 06:09:11AM -0600, Eric Blake wrote: > On 2/12/20 1:27 AM, Wouter Verhelst wrote: > >Hi, > > > >On Mon, Feb 10, 2020 at 10:52:55PM +0000, Richard W.M. Jones wrote: > >>But anyway ... could a flag indicating that the whole image is sparse > >>be useful, either as well as NBD_INIT_SPARSE or instead of it? You > >>could use it to avoid an initial disk trim, which is something that > >>mke2fs does: > > > >Yeah, I think that could definitely be useful. I honestly can't see a > >use for NBD_INIT_SPARSE as defined in this proposal; and I don't think > >it's generally useful to have a feature if we can't think of a use case > >for it (that creates added complexity for no benefit). > > > >If we can find a reasonable use case for NBD_INIT_SPARSE as defined in > >this proposal, then just add a third bit (NBD_INIT_ALL_SPARSE or > >something) that says "the whole image is sparse". Otherwise, I think we > >should redefine NBD_INIT_SPARSE to say that. > > Okay, in v2, I will start with just two bits, NBD_INIT_SPARSE > (entire image is sparse, nothing is allocated) and NBD_INIT_ZERO > (entire image reads as zero), and save any future bits for later > additions. Do we think that 16 bits is sufficient for the amount of > initial information likely to be exposed?
So as I understand the proposal, the 16 bit limit comes about because we want a round 4 byte reply, 16 bits are used by NBD_INFO_INIT_STATE and that leaves 16 bits feature bits. Therefore the only way to go from there is to have 32 feature bits but an awkward unaligned 6 byte structure, or 48 feature bits (8 byte structure). I guess given those constraints we can stick with 16 feature bits, and if we ever needed more then we'd have to introduce NBD_INFO_INIT_STATE2. The only thing I can think of which might be useful is a "fully preallocated" bit which might be used as an indication that writes are fast and are unlikely to fail with ENOSPC. > Are we in agreement that > my addition of an NBD_INFO_ response to NBD_OPT_GO is the best way > to expose initial state bits? Seems reasonable to me. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW