[ClusterLabs] A word of warning regarding VirtualDomain and utilization

[Ken Gaillot]

On Fri, 2020-12-11 at 11:59 +0100, Ulrich Windl wrote:
> Hi!
> 
> A word of warning (for SLES15 SP2):
> I learned that the VirtualDomain RA sets utilization parameters "cpu"
> and "hv_memory" for the resource's permanent configuration. That is:
> You'll see those parameters, even though you did not configure them.
> So far, so good, but when you defined a utilization-based placement
> strategy like placement-strategy=balanced, the VMs won't start as
> nobody did define those node capacities.
> 
> Questions at that point:
> Should the resource manager warn if a utilization variable is queried
> when it's not defined (I mean when it has no value at all, different
> from being zero)?
> Why can't crm shell "configure show property" or show any property by
> name? The only way to see them is just "show" which shows
> "everything". Maybe I don't know.
> 
> Continuing with the problem:
> The only RA that sets those utilization values is NodeUtilization. So
> you'll have to configure that in order to use VirtualDomain with
> default settings.
> 
> Related question: Why can't crm_mon display (at least) node's
> utilization parameters ("remaining capacities")? The only command
> known to me to query those is "crm_simulate -LUs | grep capacity:".

That's a good suggestion. I'll add it to the (unfortunately already
long) to-do list. Feel free to open a bz as well.

> Another problem is (maybe I had talked about that a long time ago
> already) that "cpu"s are just integers: If a VM has two vcpus, then
> it will consume two physical cpus of the node.
> You cannot model any resources that need somthing like 1/2 or 1/10-th 
> of a CPU (just to help the load balancing for non-VM resources).

Actually, you can, since Pacemaker doesn't care what the units are.
Define your cpu utilization attribute as 0.1 CPUs, and there you go.

The NodeUtilization agent doesn't support that currently, but you can
define them statically instead. (It might also be a good RFE for
NodeUtilization to offer a CPU factor parameter, e.g. a factor of 10
would mean it would set the node attribute to 10 times the actual
number.)

> Regards,
> Ulrich
> 
> 
> 
> 
> 
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-- 
Ken Gaillot <kgaillot at redhat.com>