241 lines
7.5 KiB
Markdown
241 lines
7.5 KiB
Markdown
---
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title: Autoscale the DNS Service in a Cluster
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content_template: templates/task
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---
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{{% capture overview %}}
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This page shows how to enable and configure autoscaling of the DNS service in a
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Kubernetes cluster.
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{{% /capture %}}
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{{% capture prerequisites %}}
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* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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* Make sure the [DNS feature](/docs/concepts/services-networking/dns-pod-service/) itself is enabled.
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* Kubernetes version 1.4.0 or later is recommended.
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{{% /capture %}}
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{{% capture steps %}}
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## Determining whether DNS horizontal autoscaling is already enabled
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List the Deployments in your cluster in the kube-system namespace:
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kubectl get deployment --namespace=kube-system
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The output is similar to this:
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NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
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...
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kube-dns-autoscaler 1 1 1 1 ...
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...
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If you see "kube-dns-autoscaler" in the output, DNS horizontal autoscaling is
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already enabled, and you can skip to
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[Tuning autoscaling parameters](#tuning-autoscaling-parameters).
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## Getting the name of your DNS Deployment or ReplicationController
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List the Deployments in your cluster in the kube-system namespace:
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kubectl get deployment --namespace=kube-system
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The output is similar to this:
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NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
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...
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kube-dns 1 1 1 1 ...
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...
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In Kubernetes versions earlier than 1.5 DNS is implemented using a
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ReplicationController instead of a Deployment. So if you don't see kube-dns,
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or a similar name, in the preceding output, list the ReplicationControllers in
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your cluster in the kube-system namespace:
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kubectl get rc --namespace=kube-system
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The output is similar to this:
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NAME DESIRED CURRENT READY AGE
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...
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kube-dns-v20 1 1 1 ...
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...
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## Determining your scale target
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If you have a DNS Deployment, your scale target is:
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Deployment/<your-deployment-name>
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where <dns-deployment-name> is the name of your DNS Deployment. For example, if
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your DNS Deployment name is kube-dns, your scale target is Deployment/kube-dns.
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If you have a DNS ReplicationController, your scale target is:
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ReplicationController/<your-rc-name>
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where <your-rc-name> is the name of your DNS ReplicationController. For example,
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if your DNS ReplicationController name is kube-dns-v20, your scale target is
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ReplicationController/kube-dns-v20.
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## Enabling DNS horizontal autoscaling
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In this section, you create a Deployment. The Pods in the Deployment run a
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container based on the `cluster-proportional-autoscaler-amd64` image.
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Create a file named `dns-horizontal-autoscaler.yaml` with this content:
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{{< code file="dns-horizontal-autoscaler.yaml" >}}
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In the file, replace `<SCALE_TARGET>` with your scale target.
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Go to the directory that contains your configuration file, and enter this
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command to create the Deployment:
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kubectl create -f dns-horizontal-autoscaler.yaml
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The output of a successful command is:
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deployment "kube-dns-autoscaler" created
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DNS horizontal autoscaling is now enabled.
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## Tuning autoscaling parameters
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Verify that the kube-dns-autoscaler ConfigMap exists:
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kubectl get configmap --namespace=kube-system
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The output is similar to this:
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NAME DATA AGE
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...
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kube-dns-autoscaler 1 ...
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...
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Modify the data in the ConfigMap:
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kubectl edit configmap kube-dns-autoscaler --namespace=kube-system
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Look for this line:
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linear: '{"coresPerReplica":256,"min":1,"nodesPerReplica":16}'
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Modify the fields according to your needs. The "min" field indicates the
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minimal number of DNS backends. The actual number of backends number is
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calculated using this equation:
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replicas = max( ceil( cores * 1/coresPerReplica ) , ceil( nodes * 1/nodesPerReplica ) )
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Note that the values of both `coresPerReplica` and `nodesPerReplica` are
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integers.
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The idea is that when a cluster is using nodes that have many cores,
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`coresPerReplica` dominates. When a cluster is using nodes that have fewer
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cores, `nodesPerReplica` dominates.
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There are other supported scaling patterns. For details, see
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[cluster-proportional-autoscaler](https://github.com/kubernetes-incubator/cluster-proportional-autoscaler).
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## Disable DNS horizontal autoscaling
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There are a few options for turning DNS horizontal autoscaling. Which option to
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use depends on different conditions.
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### Option 1: Scale down the kube-dns-autoscaler deployment to 0 replicas
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This option works for all situations. Enter this command:
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kubectl scale deployment --replicas=0 kube-dns-autoscaler --namespace=kube-system
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The output is:
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deployment "kube-dns-autoscaler" scaled
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Verify that the replica count is zero:
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kubectl get deployment --namespace=kube-system
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The output displays 0 in the DESIRED and CURRENT columns:
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NAME DESIRED CURRENT UP-TO-DATE AVAILABLE AGE
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...
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kube-dns-autoscaler 0 0 0 0 ...
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...
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### Option 2: Delete the kube-dns-autoscaler deployment
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This option works if kube-dns-autoscaler is under your own control, which means
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no one will re-create it:
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kubectl delete deployment kube-dns-autoscaler --namespace=kube-system
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The output is:
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deployment "kube-dns-autoscaler" deleted
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### Option 3: Delete the kube-dns-autoscaler manifest file from the master node
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This option works if kube-dns-autoscaler is under control of the
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[Addon Manager](https://git.k8s.io/kubernetes/cluster/addons/README.md)'s
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control, and you have write access to the master node.
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Sign in to the master node and delete the corresponding manifest file.
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The common path for this kube-dns-autoscaler is:
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/etc/kubernetes/addons/dns-horizontal-autoscaler/dns-horizontal-autoscaler.yaml
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After the manifest file is deleted, the Addon Manager will delete the
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kube-dns-autoscaler Deployment.
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{{% /capture %}}
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{{% capture discussion %}}
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## Understanding how DNS horizontal autoscaling works
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* The cluster-proportional-autoscaler application is deployed separately from
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the DNS service.
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* An autoscaler Pod runs a client that polls the Kubernetes API server for the
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number of nodes and cores in the cluster.
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* A desired replica count is calculated and applied to the DNS backends based on
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the current schedulable nodes and cores and the given scaling parameters.
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* The scaling parameters and data points are provided via a ConfigMap to the
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autoscaler, and it refreshes its parameters table every poll interval to be up
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to date with the latest desired scaling parameters.
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* Changes to the scaling parameters are allowed without rebuilding or restarting
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the autoscaler Pod.
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* The autoscaler provides a controller interface to support two control
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patterns: *linear* and *ladder*.
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## Future enhancements
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Control patterns, in addition to linear and ladder, that consider custom metrics
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are under consideration as a future development.
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Scaling of DNS backends based on DNS-specific metrics is under consideration as
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a future development. The current implementation, which uses the number of nodes
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and cores in cluster, is limited.
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Support for custom metrics, similar to that provided by
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[Horizontal Pod Autoscaling](/docs/tasks/run-application/horizontal-pod-autoscale/),
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is under consideration as a future development.
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{{% /capture %}}
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{{% capture whatsnext %}}
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Learn more about the
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[implementation of cluster-proportional-autoscaler](https://github.com/kubernetes-incubator/cluster-proportional-autoscaler).
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{{% /capture %}}
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