Re: [PATCH] libxl: add option for discard support to xl disk configuration

[Olaf Hering <olaf@aepfle.de>]

On Wed, Jan 29, Ian Campbell wrote:

> On Wed, 2014-01-29 at 12:19 +0100, Olaf Hering wrote:
> > The toolstack just does not know if a phy device supports it, or if file
> > backed storage can do hole punching. If feature-discard is set and the
> > frontend sends a discard request, the backend would return an error
> > (like ENOTSUPPORTED) and the frontend internally disables the discard
> > flag. Thats how it is done in pvops and the forward ported xenlinux
> > tree.
> 
> That sounds good.
> 
> Is it worth noting that enable-discard=1 is only advisory and will be
> ignored if the underlying storage and/or backend doesn't understand it?
> The real benefit of this option is to be able to force it off rather
> than on I think.

Yes, the purpose is to turn it off. I will extend the description.

> > Thats what I'm asking you. Why is readwrite set here, and later on also
> > in the .l file? At least just setting it here did not unconditionally
> > enable it if no discard= was specified. I have not traced the code why
> > that happens.
> 
> One for Ian J I think. Perhaps it is just setting the default?

For some reason this setting is lost, at least for my discard flag.  The
readwrite part may suffer from the same issue, otherwise access_set
could be removed.  I will trace the code to understand how it works.

Olaf