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:
> 
> > 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?
> 
> If the backing file is fully ppopulated on purpose to avoid
> fragmentation of that file, then silently enabling discard means the
> unwanted fragmentation will occour over time. If anyone is really doing
> that in their setup I certainly dont know. At least kvm/qemu has such
> knob, and also an hyperv host offers sparse, fully populated and overlay
> as possible option when creating a new virtual disk image.
> 
> Now I think that if libxl already writes feature-discard=0, qemu can use
> that to clear BDRV_O_UNMAP. Also blkbk could be extended to first check
> if the property already exists before blindly enabling discard. No idea
> if that makes sense for "phy", but you never know.

OK, but it is important that we don't collapse the two options into one:
the backend might or might not support feature-discard so we can't have
the toolstack write feature-discard directly.

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