Add options to control images pulling of kubelet

Signed-off-by: tu1h <[email protected]>

docs/vars.md

@@ -216,6 +216,10 @@ Stack](https://github.com/kubernetes-sigs/kubespray/blob/master/docs/dns-stack.m
  The percent is calculated by dividing this field value by 100, so the field value must be between 0 and 100, inclusive.
  When specified, the value must be less than imageGCHighThresholdPercent. Default: 80
 
+* *kubelet_serialize_image_pulls* - If `true`, the Kubelet pulls images one at a time. We recommend not set to false on nodes that an Aufs storage backend.
+
+* *kubelet_max_parallel_image_pulls* - Sets the maximum number of image pulls in parallel. This field cannot be set if *kubelet_serialize_image_pulls* is true. Setting it to `nil` means no limit.
+
 * *kubelet_make_iptables_util_chains* - If `true`, causes the kubelet ensures a set of `iptables` rules are present on host.
 
 * *kubelet_cpu_manager_policy* - If set to `static`, allows pods with certain resource characteristics to be granted increased CPU affinity and exclusivity on the node. And it should be set with `kube_reserved` or `system-reserved`, enable this with the following guide:[Control CPU Management Policies on the Node](https://kubernetes.io/docs/tasks/administer-cluster/cpu-management-policies/)

roles/kubernetes/node/defaults/main.yml

@@ -263,3 +263,8 @@ conntrack_modules:
 kubelet_tracing: false
 kubelet_tracing_endpoint: 0.0.0.0:4317
 kubelet_tracing_sampling_rate_per_million: 100
+
+# Whether pull images one at a time
+kubelet_serialize_image_pulls: true
+# The maximum number of image pulls in parallel. This option take effect if kubelet_serialize_image_pulls is false
+kubelet_max_parallel_image_pulls: 5

roles/kubernetes/node/templates/kubelet-config.v1beta1.yaml.j2

@@ -174,4 +174,8 @@ topologyManagerScope: {{ kubelet_topology_manager_scope }}
 tracing:
  endpoint: {{ kubelet_tracing_endpoint }}
  samplingRatePerMillion: {{ kubelet_tracing_sampling_rate_per_million }}
+{% endif %}
+serializeImagePulls: {{ kubelet_serialize_image_pulls | bool | lower }}
+{% not kubelet_serialize_image_pulls %}
+maxParallelImagePulls: {{ kubelet_max_parallel_image_pulls }}
 {% endif %}