Re: [Qemu-devel] [PATCH v2] make windows notice media change

[Stefano Stabellini]

On Wed, 29 Jul 2009, Paul Brook wrote:
> > >> BTW, why would there be the version parameter in the first place if it
> > >> wasn't supposed to load older versions?!
> > >
> > > Like I already said: it's there to prevent an old version being loaded
> > > accidentally. Without this an incompatible change will  result in
> > > anything from a crash to corrupt/inconsistent guest state. Versioning
> > > allows us to reject the snapshot and fail safely.
> >
> > If that was the case then the if (version != x) return -EINVAL check could
> > have been in the generic code and there would be no need for the version
> > parameter in the load function.
> 
> Preventing loading bad snapshots is the primary goal.
> 
> Allowing loading old snapshots is a secondary feature. Personally I think 
> it's 
> not worth the effort, and in practice is unlikely to be feasible for whole 
> machines over any significant length of time. However I don't feel strongly 
> enough to actually rip the code out.
> 

I completely disagree with you: loading old snapshots is a really
important feature, at least for xen (and kvm) users.
Think about a sysadmin that kept his server running for the last X years
in a VM using xen (or kvm) and now wants to upgrade his host.
Is he supposed to reboot the VM as well?
Are we seriously suggesting that everytime the hypervisor is upgraded
the sysadmin must reboot all his VMs?

Regarding the difficulty of the task: we (xen community and xenserver)
accomplished this for the last 4 years with no particular problems and a
relatively small developing team.
I have a patch in my patchqueue that allows a modern qemu to load snapshot
from version 1 and it doesn't require the use of black magic.