private double computeGroupAvailability(Group g) {
            // TODO also look at distributors
            final long availableNodes = availableNodesIn(g).count();
            // Model should make it impossible to deploy with zero nodes in a group,
            // so no div by zero risk.
            return availableNodes / (double)g.getNodes().size();
        }
If the number of nodes safely set to maintenance is at least this number, the remaining
nodes in the group will be set to maintenance (storage nodes) or down (distributors).

<p>This feature is disabled if safeMaintenanceGroupThreshold is 0 (not default).</p>
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/GroupAvailabilityCalculator.java