Re: [PATCH 2 of 3] interface: Flesh out the BLKIF_OP_DISCARD description

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

On Thu, Oct 13, 2011 at 09:00:07AM +0100, Ian Campbell wrote:
> Thanks for splitting these out.
> 
> On Wed, 2011-10-12 at 23:12 +0100, Konrad Rzeszutek Wilk wrote:
> [...]
> > + * The backend can optionally provide two extra XenBus attributes to
> > + * further optimize the discard functionality:
> > + * 'discard-aligment' - Devices that support discard functionality may
> > + * internally allocate space in units that are bigger than the exported
> > + * logical block size. The discard-alignment parameter indicates how many bytes
> > + * the beginning of the partition is offset from the internal allocation unit's
> > + * natural alignment.
> 
> So this is to account for the case where a physical device can discard
> e.g. 128K blocks at a time but the VBD (a better term than "partition"
> in the context, I think) starts at e.g. offset 64K within that
> underlying device?
> 
> Does this mean that the virtual device can discard the first 64K (and
> thereafter in 128K chunks), or that it cannot because that would overlap

[edit: I don't think I answered this question]
Yes.
> the first 64K of that block which belongs to something else? Or that it
> can try but it may or may not succeed. What about if the secure flag is