Re: [Xen-devel] [PATCH 2/2] expert_mode: Add a new configuration option for expert users.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jul 17, 2013 at 11:56:24AM +0100, Ian Jackson wrote:
> Ian Campbell writes ("Re: [Xen-devel] [PATCH 2/2] expert_mode: Add a new 
> configuration option for expert users."):
> > On Fri, 2013-07-12 at 14:08 -0400, Konrad Rzeszutek Wilk wrote:
> > > This could also be called 'seatbelt' option.
> > > 
> > > libxl has a variety of checks where it will fail out an operation
> > > unless the user has provided an --force (or --ignore) parameter.
> > > Currently one such check is for the 'vcpu-set' command which
> > > will error out if the count of virtual cpus is greater than the
> > > physical cpus. This parameter will ignore such checks and allow
> > > the user to do the operations without the need for override flags.
> > 
> > Does this overlap  somewhat with various commands which individually
> > take a -f(orce) option?
> 
> Clearly it should disable all of those -f's too.
> 
> > > +=item B<expert_mode=BOOLEAN>
> > > +
> > > +Do not act on host performed checks that might lead to performance
> > > +degradations. Currently checks are made for following operations:
> > > + - C<vcpu-set> - if the number of VCPUs set for a guest is higher than 
> > > the
> > > +   physical count the operation will error out.
> 
> I don't think this is a very coherent specification.  Surely it should
> override "all -f options" or something similar.

Sure, I hadn't looked at the other ones.
> 
> I still don't see why you would want such a thing.

We discussed it during Xen 4.3 release that certain changes, like this one
are inconsistent (for example you can launch an guest with more vCPUs
than pCPU, but you if you lower the amount of vCPUs you can't increase
it until you use -f flag). 

But you could consider such inconsistent behavior to be a failsafe
mechanism so that the user does not do something silly. But if they
are an expert... well

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