[Libguestfs] Cross-project NBD extension proposal: NBD_INFO_INIT_STATE

[Richard W.M. Jones]

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.

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