Re: [Qemu-devel] [RFC] Re-evaluating subcluster allocation for qcow2 images

[Kevin Wolf]

Am 28.06.2019 um 17:10 hat Alberto Garcia geschrieben:
> On Fri 28 Jun 2019 05:03:13 PM CEST, Denis Lunev wrote:
> > On 6/28/19 6:02 PM, Alberto Garcia wrote:
> >>>>> Please note, I am not talking now about your case with COW. Here the
> >>>>> allocation is performed on the sub-cluster basis, i.e. the abscence of
> >>>>> the sub-cluster in the image means hole on that offset. This is
> >>>>> important difference.
> >>>> I mentioned the possibility that if you have a case like 2MB / 64KB
> >>>> and you write to an empty cluster then you could allocate the
> >>>> necessary subclusters, and additionally fallocate() the space of the
> >>>> whole cluster (2MB) in order to try to keep it contiguous.
> >>>>
> >>>> With this we would lose the space saving advantage of having
> >>>> subclusters. But perhaps that would work for smaller cluster sizes
> >>>> (it would mitigate the fragmentation problem).
> >>> There seem to be use cases for both ways. So does this need to be an
> >>> option?
> >> Probably a runtime option, or a heuristic that decides what to do
> >> depending on the cluster size.
> > no, I think that this should be on-disk option as this affects
> > allocation strategy.
> 
> But why does it need to be stored on-disk? It should be theoretically
> possible to switch between on strategy and the other at runtime (not
> that it would make sense though).

I think it makes sense to store the default in the image and allow it to
be overridden at runtime, similar to lazy_refcounts.

Kevin