[ClusterLabs] Resource balancing and "ptest scores"

[Ken Gaillot]

On Wed, 2021-02-24 at 11:16 +0100, Ulrich Windl wrote:
> Hi!
> 
> Using a utilization-based placement strategy (placement-
> strategy=balanced), I wonder why pacemaker chose node h16 to place a
> new resource.
> 
> The situation before placement looks like this:
> Remaining: h16 capacity: utl_ram=207124 utl_cpu=340
> Remaining: h18 capacity: utl_ram=209172 utl_cpu=360
> Remaining: h19 capacity: utl_ram=180500 utl_cpu=360
> 
> So h18 has most resources left.
> 
> A new resource prm_xen_v16 (utilization utl_cpu=40 utl_ram=16384)
> will be placed to h16, however, and I don't understand why:
> 
> Transition Summary:
>  * Start      prm_xen_v16           ( h16 )
>  * Start      prm_cron_snap_v16     ( h16 )
> 
> (the "snap" resource depends on the xen resource)
> 
> The cluster actually placed the resource as indicated, leaving:
> Remaining: h16 capacity: utl_ram=190740 utl_cpu=300
> Remaining: h18 capacity: utl_ram=209172 utl_cpu=360
> Remaining: h19 capacity: utl_ram=180500 utl_cpu=360
> 
> So h18 still has most capacity left.
> 
> I have 5 VMs on h16, 3 VMs on h18, and 2 VMs on h19...
> 
> pacemaker-2.0.4+20200616.2deceaa3a-3.3.1.x86_64 of SLES15 SP2.
> 
> Regards,
> Ulrich

Pacemaker checks the resource's node scores first (highest score wins
the resource -- assuming the node has sufficient capacity of course).
Only if node scores are equal will it choose based on free capacity.

For example, if a location constraint gives a particular node a high
enough preference, that will be considered more important than free
capacity. ("High enough" being relative to the rest of the
configuration -- other constraint scores, etc.)
-- 
Ken Gaillot <kgaillot at redhat.com>