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

[Olaf Hering <olaf@aepfle.de>]

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.

> > blkfront_setup_discard should check for "qdisk" instead of (, or in
> > addition to?) "file" to actually make use of this new feature.
> 
> Why? I don't think that would be correct: if the feature is advertised
> on xenstore by the backend (feature-discard) then blkfront can/should
> use it. If it is not present then it is not going to use it.
> Let's not complicate things further.

blockfront is broken:
http://lists.xenproject.org/archives/html/xen-devel/2014-01/msg00988.html


> > +++ b/hw/block/xen_disk.c
> > @@ -68,6 +68,8 @@ struct ioreq {
> >      int                 presync;
> >      int                 postsync;
> >      uint8_t             mapped;
> > +    int64_t             sector_num;
> > +    int                 nb_sectors;
> 
> You have access to the original request via req, I don't think you need
> these two fields, do you?

I will double check if thats doable.

Thanks,

Olaf