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Re: [Xen-devel] blkif discard attributes

On Thu, Jul 03, 2014 at 11:24:30AM +0100, Jan Beulich wrote:
> >>> On 03.07.14 at 12:17, <david.vrabel@xxxxxxxxxx> wrote:
> > On 03/07/14 11:08, Jan Beulich wrote:
> >>>>> On 03.07.14 at 11:43, <david.vrabel@xxxxxxxxxx> wrote:
> >>> On 03/07/14 08:59, Jan Beulich wrote:
> >>>> discard_zeroes_data also be communicated from backend to frontend?
> >>>
> >>> Perhaps. But how would you handle a guest that used to have
> >>> discard_zeroes_data but is restored with different storage that no
> >>> longer has this property?
> >> 
> >> Don't these attributes gets re-evaluated after restore anyway?
> > 
> > I don't see how that helps.  What if a discard request was submitted
> > before the suspend, expecting the discard to zero and on restore the
> > requests is queued for the new backend which then might not zero.
> Hmm, good point. So it's then indeed better to not communicate this.
> Albeit similar issues arise with the existing attributes we communicate:
> What if the new backend doesn't satisfy the assumptions on the old
> one? Possibly the request may get failed, but possibly it may also get
> executed wrongly. Neither of which is very desirable.

That is OK with discard operations. It is OK if they fail intermediately.
> Jan

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