Re: [Xen-devel] [PATCH] [RFC] qemu-upstream: add discard support for xen_disk

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 17 Jan 2014, Olaf Hering wrote:
> On Fri, Jan 17, Stefano Stabellini wrote:
> 
> > On Fri, 17 Jan 2014, Olaf Hering wrote:
> > > On Fri, Jan 17, Stefano Stabellini wrote:
> > > 
> > > > On Thu, 9 Jan 2014, Olaf Hering wrote:
> > > > > The discard support is enabled unconditionally. But it would be worth 
> > > > > to
> > > > > have a knob to disable it in case the backing file was intentionally
> > > > > created non-sparse to avoid fragmentation.
> > > > > How could this be knob be passed from domU.cfg:disk=[] to the actual
> > > > > qemu process?
> > > > 
> > > > It would need to be on xenstore, because that is the only per-disk
> > > > interface xen_disk is listening to.
> > > 
> > > I figured that out. There are already script=, backend= and other knobs.
> > > I will see how to add a discard=on|off to libxl and write that to the
> > > xenstore backend node so qemu can get it from there.
> > > What property name do you suggest? I have something like
> > > "toolstack-option-discard" in mind.
> > 
> > discard_enabled?
> 
> Isnt that name too generic? In the end that node is used also by backend
> and frontend.

The problem is that it is confusing to have two options in the same
place, one written by the toolstack for the backend and the other
written by the backend for the frontend.

Can't we just assume that if the backend can do discard on that file, it
is simply going to enable feature-discard? Do we really need the
toolstack driven option too?

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