[Libguestfs] Cross-project NBD extension proposal: NBD_INFO_INIT_STATE

[Eric Blake]

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?  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?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org