private static NodeResources.DiskSpeed parseOptionalDiskSpeed(String diskSpeedString) {
        if (diskSpeedString == null) return NodeResources.DiskSpeed.getDefault();
        return switch (diskSpeedString) {
            case "fast" -> NodeResources.DiskSpeed.fast;
            case "slow" -> NodeResources.DiskSpeed.slow;
            case "any" -> NodeResources.DiskSpeed.any;
            default -> throw new IllegalArgumentException("Illegal disk-speed value '" + diskSpeedString +
                                                          "': Legal values are 'fast', 'slow' and 'any')");
        };
    }
Returns whether the count attribute was present on the {@code <nodes>} element.
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/NodesSpecification.java