README: Add note about cluster-autoscaler not supporting multiple AZs

README.md

@@ -130,7 +130,7 @@ To use a 3-5 node Auto Scaling Group, run:
 eksctl create cluster --name=cluster-5 --nodes-min=3 --nodes-max=5
 ```
 
-> NOTE: You will still need to install and configure autoscaling. See the "Enable Autoscaling" section below.
+> NOTE: You will still need to install and configure autoscaling. See the "Enable Autoscaling" section below. Also note that depending on your workloads you might need to use a separate nodegroup for each AZ. See [Zone-aware Autoscaling](#zone-aware-autoscaling) below for more info.
 
 To use 30 `c4.xlarge` nodes and prevent updating current context in `~/.kube/config`, run:
 
@@ -410,6 +410,43 @@ and `k8s.io/cluster-autoscaler/<clusterName>` tags, so nodegroup discovery shoul
 
 [cluster autoscaler]: https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/cloudprovider/aws/README.md
 
+#### Zone-aware Autoscaling
+
+If your workloads are zone-specific you'll need to create separate nodegroups for each zone.  This is because the `cluster-autoscaler` assumes that all nodes in a group are exactly equivalent.  So, for example, if a scale-up event is triggered by a pod which needs a zone-specific PVC (e.g. an EBS volume), the new node might get scheduled in the wrong AZ and the pod will fail to start.
+
+You won't need a separate nodegroup for each AZ if your environment meets the following criteria:
+
+- The AZRebalance scaling process is [suspended](https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-suspend-resume-processes.html).
+- No zone-specific storage requirements.
+- No podAffinity with topology other than host.
+- No nodeAffinity on zone label.
+- Never scale any zone to 0.
+
+(Read more [here](https://github.com/kubernetes/autoscaler/pull/1802#issuecomment-474295002) and [here](https://github.com/weaveworks/eksctl/pull/647#issuecomment-474698054).)
+
+If you meet all of the above requirements (and possibly others) then you should be safe with a single nodegroup which spans multiple AZs.  Otherwise you'll want to create separate, single-AZ nodegroups:
+
+BEFORE:
+
+```yaml
+nodeGroups:
+  - name: ng1-public
+    instanceType: m5.xlarge
+    # availabilityZones: ["eu-west-2a", "eu-west-2b"]
+```
+
+AFTER:
+
+```yaml
+nodeGroups:
+  - name: ng1-public-2a
+    instanceType: m5.xlarge
+    availabilityZones: ["eu-west-2a"]
+  - name: ng1-public-2b
+    instanceType: m5.xlarge
+    availabilityZones: ["eu-west-2b"]
+```
+
 ### VPC Networking
 
 By default, `eksctl create cluster` will build a dedicated VPC, in order to avoid interference with any existing resources for a