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Cluster Autoscaler on AWS

The cluster autoscaler on AWS scales worker nodes within any specified autoscaling group. It will run as a Deployment in your cluster. This README covers the steps required to configure and run the cluster autoscaler.

Kubernetes Version

Cluster autoscaler must run on v1.3.0 or greater.

Permissions

The worker running the cluster autoscaler will need access to certain resources and actions. We always recommend you to attach IAM policy to nodegroup and avoid using AWS credentials directly unless you have special requirements.

Attach IAM policy to NodeGroup

A minimum IAM policy would look like:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "autoscaling:DescribeAutoScalingGroups",
                "autoscaling:DescribeAutoScalingInstances",
                "autoscaling:DescribeLaunchConfigurations",
                "autoscaling:SetDesiredCapacity",
                "autoscaling:TerminateInstanceInAutoScalingGroup"
            ],
            "Resource": "*"
        }
    ]
}

If you'd like to scale node groups from 0, an autoscaling:DescribeLaunchConfigurations or ec2:DescribeLaunchTemplateVersions permission is required depending on if you made your ASG with Launch Configuration or Launch Template.

If you'd like the cluster autoscaler to automatically discover EC2 AutoScalingGroups, the autoscaling:DescribeTags permission is also required.

NOTE: You can restrict the target resources for the autoscaling actions by specifying autoscaling group ARNS. More information can be found here.

Using AWS Credentials

For on premise users wishing to scale out to AWS, the above approach of attaching policy to a nodegroup role won't work. Instead, you can create an aws secret manually and add following environment variables to cluster-autoscaler deployment manifest. Cluster autoscaler will use credential to authenticate and authorize itself. Please make sure your role has above permissions.

apiVersion: v1
kind: Secret
metadata:
  name: aws-secret
type: Opaque
data:
  aws_access_key_id: BASE64_OF_YOUR_AWS_ACCESS_KEY_ID
  aws_secret_access_key: BASE64_OF_YOUR_AWS_SECRET_ACCESS_KEY

Please check guidance for creating a secret manually.

env:
- name: AWS_ACCESS_KEY_ID
  valueFrom:
    secretKeyRef:
      name: aws-secret
      key: aws_access_key_id
- name: AWS_SECRET_ACCESS_KEY
  valueFrom:
    secretKeyRef:
      name: aws-secret
      key: aws_secret_access_key
- name: AWS_REGION
  value: YOUR_AWS_REGION

Deployment Specification

Auto-Discovery Setup is always preferred option to avoid multiple, potentially different configuration for min/max values. If you want to adjust minimum and maximum size of the group, please adjust size on ASG directly, CA will fetch latest change when talking to ASG.

If you use one or multiple ASG setup, the min/max configuration change in CA will not make the corresponding change to ASG. Please make sure CA min/max values are within the boundary of ASG minSize and maxSize.

One ASG Setup (min: 1, max: 10, ASG Name: k8s-worker-asg-1)

kubectl apply -f examples/cluster-autoscaler-one-asg.yaml

Multiple ASG Setup

kubectl apply -f examples/cluster-autoscaler-multi-asg.yaml

Master Node Setup

To run a CA pod in master node - CA deployment should tolerate the master taint and nodeSelector should be used to schedule the pods in master node. Please replace {{ node_asg_min }}, {{ node_asg_max }} and {{ name }} with your ASG setting in the yaml file.

kubectl apply -f examples/cluster-autoscaler-run-on-master.yaml

Auto-Discovery Setup

To run a cluster-autoscaler which auto-discovers ASGs with nodes use the --node-group-auto-discovery flag. For example, --node-group-auto-discovery=asg:tag=k8s.io/cluster-autoscaler/enabled,k8s.io/cluster-autoscaler/<YOUR CLUSTER NAME> will find the ASGs where those tag keys exist. It does not matter what value the tags have.

Note that:

  • It is recommended to use a second tag like k8s.io/cluster-autoscaler/<YOUR CLUSTER NAME> when k8s.io/cluster-autoscaler/enabled is used across many clusters to prevent ASGs from different clusters recognized as the node groups
  • There are no --nodes flags passed to cluster-autoscaler because the node groups are automatically discovered by tags
  • No min/max values are provided when using Auto-Discovery, cluster-autoscaler will respect the current min and max values of the ASG being targeted, and it will adjust only the "desired" value.
kubectl apply -f examples/cluster-autoscaler-autodiscover.yaml

Scaling a node group to 0

From CA 0.6.1 - it is possible to scale a node group to 0 (and obviously from 0), assuming that all scale-down conditions are met.

If you are using nodeSelector you need to tag the ASG with a node-template key "k8s.io/cluster-autoscaler/node-template/label/" and "k8s.io/cluster-autoscaler/node-template/taint/" if you are using taints.

For example for a node label of foo=bar you would tag the ASG with:

{
    "ResourceType": "auto-scaling-group",
    "ResourceId": "foo.example.com",
    "PropagateAtLaunch": true,
    "Value": "bar",
    "Key": "k8s.io/cluster-autoscaler/node-template/label/foo"
}

And for a taint of "dedicated": "foo:NoSchedule" you would tag the ASG with:

{
    "ResourceType": "auto-scaling-group",
    "ResourceId": "foo.example.com",
    "PropagateAtLaunch": true,
    "Value": "foo:NoSchedule",
    "Key": "k8s.io/cluster-autoscaler/node-template/taint/dedicated"
}

If you'd like to scale node groups from 0, an autoscaling:DescribeLaunchConfigurations or ec2:DescribeLaunchTemplateVersions permission is required depending on if you made your ASG with Launch Configuration or Launch Template:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "autoscaling:DescribeAutoScalingGroups",
                "autoscaling:DescribeAutoScalingInstances",
                "autoscaling:DescribeTags",
                "autoscaling:DescribeLaunchConfigurations",
                "autoscaling:SetDesiredCapacity",
                "autoscaling:TerminateInstanceInAutoScalingGroup",
                "ec2:DescribeLaunchTemplateVersions"
            ],
            "Resource": "*"
        }
    ]
}

Using AutoScalingGroup MixedInstancesPolicy

Note: The minimum version of cluster autoscaler to support MixedInstancePolicy is v1.14.x.

It is possible to use Cluster Autoscaler with a mixed instances policy, to enable diversification across on-demand and spot instances, of multiple instance types in a single ASG. When using spot instances, this increases the likelihood of successfully launching a spot instance to add the desired capacity to the cluster versus a single instance type, which may be in short supply.

Note that the instance types should have the same amount of RAM and number of CPU cores, since this is fundamental to CA's scaling calculations. Using mismatched instances types can produce unintended results.

Additionally, there are other factors which affect scaling, such as node labels. If you are currently using nodeSelector with the beta.kubernetes.io/instance-type label, you will need to apply a common propagating label to the ASG and use that instead, since the instance-type label can no longer be relied upon. One may also use auto-generated tags such as aws:cloudformation:stack-name for this purpose. Node affinity and anti-affinity are not affected in the same way, since these selectors natively accept multiple values; one must add all the configured instances types to the list of values, for example:

spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: beta.kubernetes.io/instance-type
            operator: In
            values:
            - r5.2xlarge
            - r5d.2xlarge
            - i3.2xlarge
            - r5a.2xlarge
            - r5ad.2xlarge

Example usage:

  • Create a Launch Template (LT) with an instance type, for example, r5.2xlarge. Consider this the 'base' instance type. Do not define any spot purchase options here.
  • Create an ASG with a MixedInstancesPolicy that refers to the newly-created LT.
  • Set LaunchTemplateOverrides to include the 'base' instance type r5.2xlarge and suitable alternatives, e.g. r5d.2xlarge, i3.2xlarge, r5a.2xlarge and r5ad.2xlarge. Differing processor types and speeds should be evaluated depending on your use-case(s).
  • Set InstancesDistribution according to your needs.
  • See Allocation Strategies for information about the ASG fulfils capacity from the specified instance types.
  • Repeat by creating other LTs and ASGs, for example c5.18xlarge and c5n.18xlarge or a bunch of similar burstable instances.

See CloudFormation example here.

Use Static Instance List

The set of the latest supported EC2 instance types will be fetched by the CA at run time. You can find all the available instance types in the CA logs. If your network access is restricted such that fetching this set is infeasible, you can specify the command-line flag --aws-use-static-instance-list=true to switch the CA back to its original use of a statically defined set.

To refresh static list, please run go run ec2_instance_types/gen.go under cluster-autoscaler/cloudprovider/aws/ and update staticListLastUpdateTime in aws_util.go

Common Notes and Gotchas:

  • The /etc/ssl/certs/ca-bundle.crt should exist by default on ec2 instance in your EKS cluster. If you use other cluster privision tools like kops with different operating systems other than Amazon Linux 2, please use /etc/ssl/certs/ca-certificates.crt or correct path on your host instead for the volume hostPath in your cluster autoscaler manifest.
  • Cluster autoscaler does not support Auto Scaling Groups which span multiple Availability Zones; instead you should use an Auto Scaling Group for each Availability Zone and enable the --balance-similar-node-groups feature. If you do use a single Auto Scaling Group that spans multiple Availability Zones you will find that AWS unexpectedly terminates nodes without them being drained because of the rebalancing feature.
  • EBS volumes cannot span multiple AWS Availability Zones. If you have a Pod with Persistent Volume in an AZ, It must be running on a k8s/EKS node which is in the same Availability Zone of the Persistent Volume. If AWS Auto Scaling Group launches a new k8s/EKS node in different AZ and moves this Pod into the new node, The Persistent volume in previous AZ will not be available from the new AZ. The pod will stay in Pending status. The Workaround is using a single AZ for the k8s/EKS nodes.
  • By default, cluster autoscaler will not terminate nodes running pods in the kube-system namespace. You can override this default behaviour by passing in the --skip-nodes-with-system-pods=false flag.
  • By default, cluster autoscaler will wait 10 minutes between scale down operations, you can adjust this using the --scale-down-delay-after-add, --scale-down-delay-after-delete, and --scale-down-delay-after-failure flag. E.g. --scale-down-delay-after-add=5m to decrease the scale down delay to 5 minutes after a node has been added.
  • If you're running multiple ASGs, the --expander flag supports three options: random, most-pods and least-waste. random will expand a random ASG on scale up. most-pods will scale up the ASG that will schedule the most amount of pods. least-waste will expand the ASG that will waste the least amount of CPU/MEM resources. In the event of a tie, cluster autoscaler will fall back to random.
  • If you're managing your own kubelets, they need to be started with the --provider-id flag. The provider id has the format aws:///<availability-zone>/<instance-id>, e.g. aws:///us-east-1a/i-01234abcdef.