Document node capacity.

docs/node.md

@@ -149,5 +149,36 @@ unschedulable, run this command:
 kubectl update nodes 10.1.2.3 --patch='{"apiVersion": "v1", "unschedulable": true}'
 ```
 
+### Node capacity
+
+The capacity of the node (number of cpus and amount of memory) is part of the node resource.
+Normally, nodes register themselves and report their capacity when creating the node resource.  If
+you are doing [manual node administration](#manual-node-administration), then you need to set node
+capacity when adding a node.
+
+The kubernetes scheduler ensures that there are enough resources for all the pods on a node.  It
+checks that the sum of the limits of containers on the node is less than the node capacity.  It
+includes all containers started by kubelet, but not containers started directly by docker, nor
+processes not in containers.  
+
+If you want to explicitly reserve resources for non-Pod processes, you can create a placeholder
+pod.  Use the following template:
+```
+apiVersion: v1
+kind: Pod
+metadata:
+  name: resource-reserver
+spec:
+  containers:
+  - name: sleep-forever
+    image: gcr.io/google_containers/pause:0.8.0
+    resources:
+      limits:
+        cpu: 100m
+        memory: 100Mi
+```
+Set the `cpu` and `memory` values to the amount of resources you want to reserve.
+Place the file in the manifest directory (`--config=DIR` flag of kubelet).  Do this
+on each kubelet where you want to reserve resources.
 
 [![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/docs/node.md?pixel)]()