Re: [PATCH] blkif.h: enhance comments related to the discard feature

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Jan 23, 2014 at 08:19:40AM +0000, Jan Beulich wrote:
> >>> On 22.01.14 at 22:14, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> > On Fri, Jan 17, 2014 at 03:09:13PM +0000, Jan Beulich wrote:
> >> >>> On 14.01.14 at 22:57, Olaf Hering <olaf@aepfle.de> wrote:
> >> > @@ -323,9 +324,14 @@
> >> >   *     For full interoperability, block front and backends should publish
> >> >   *     identical ring parameters, adjusted for unit differences, to the
> >> >   *     XenStore nodes used in both schemes.
> >> > - * (4) Devices that support discard functionality may internally allocate
> >> > - *     space (discardable extents) in units that are larger than the
> >> > - *     exported logical block size.
> >> > + * (4) Devices that support discard functionality may internally allocate 
> >> > space
> >> > + *     (discardable extents) in units that are larger than the exported 
> >> > logical
> >> > + *     block size. If the backing device has such discardable extents the
> >> > + *     backend must provide both discard-granularity and discard-alignment.
> >                     ^^^^ - MAY
> 
> I think the intention is to say that these two should go together,
> i.e. specifying just one of them is a mistake.

The 'and' in that sentence covers that I think?

My reading with 'must' is that 'features-discard' MUST have both
discard-granularity and discard-alignment. But that is not the case
- even if the device does support them - it does not have to
expose them.

> 
> Jan
> 
> >> > + *     Backends supporting discard should include discard-granularity and
> >                                         ^^^^^ - MAY
> >> > + *     discard-alignment even if it supports discarding individual sectors.
> >> > + *     Frontends should assume discard-alignment == 0 and discard-granularity 
> >> > ==
> >> > + *     sector size if these keys are missing.
> >> >   * (5) The discard-alignment parameter allows a physical device to be
> >> >   *     partitioned into virtual devices that do not necessarily begin or
> >> >   *     end on a discardable extent boundary.
> 
>