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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.01.14 at 22:14, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> On Fri, Jan 17, 2014 at 03:09:13PM +0000, Jan Beulich wrote:
>> >>> On 14.01.14 at 22:57, Olaf Hering <olaf@xxxxxxxxx> 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.

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.



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