Re: [Xen-devel] blk[front|back] does not hand over disk parameters

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Mon, 2011-02-28 at 10:55 +0000, Jan Beulich wrote:
> >>> On 28.02.11 at 11:06, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > This is not the sort of thing which changes dynamically across the
> > lifetime of a device, is it? In which case it seems like the sort of
> > information which the backend could communicate to the frontend via
> > xenbus at start of day. e.g. take a look at how the sector-size is
> > passed through xenbus. 
> > 
> > It should be trivial to add this in a compatible manner since the
> > frontend can just do what it does today if the nodes are missing and the
> > backend wouldn't rely on the frontend doing anything useful with the
> > information anyway.
> 
> Am I right in understanding that these numbers aren't used by
> the block layer itself at all, but just get provided to userspace for
> whatever optimization it can do?
> 
> Confused, Jan

I had inferred from Adi's bringing them up that the kernel would
actually use them in some way, but I don't actually know if that's the
case...

> In that case, I can't really see
> how passing through these values can really help general
> performance (i.e. for apps not paying attention to these values).

Even their utility is only if userspace explicitly makes use of them
they are just as useful in a Xen domU as they are in a non-Xen system,
so why would we not plumb them through?

Ian.



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