238 lines
6.8 KiB
Markdown
238 lines
6.8 KiB
Markdown
---
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title: Autoscale the DNS Service in a Cluster
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content_type: task
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---
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<!-- overview -->
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This page shows how to enable and configure autoscaling of the DNS service in
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your Kubernetes cluster.
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## {{% heading "prerequisites" %}}
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* {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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* This guide assumes your nodes use the AMD64 or Intel 64 CPU architecture.
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* Make sure [Kubernetes DNS](/docs/concepts/services-networking/dns-pod-service/) is enabled.
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<!-- steps -->
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## Determine whether DNS horizontal autoscaling is already enabled {#determining-whether-dns-horizontal-autoscaling-is-already-enabled}
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List the {{< glossary_tooltip text="Deployments" term_id="deployment" >}}
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in your cluster in the kube-system {{< glossary_tooltip text="namespace" term_id="namespace" >}}:
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```shell
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kubectl get deployment --namespace=kube-system
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```
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The output is similar to this:
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NAME READY UP-TO-DATE AVAILABLE AGE
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...
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dns-autoscaler 1/1 1 1 ...
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...
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If you see "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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## Get the name of your DNS Deployment {#find-scaling-target}
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List the DNS deployments in your cluster in the kube-system namespace:
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```shell
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kubectl get deployment -l k8s-app=kube-dns --namespace=kube-system
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```
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The output is similar to this:
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NAME READY UP-TO-DATE AVAILABLE AGE
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...
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coredns 2/2 2 2 ...
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...
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If you don't see a Deployment for DNS services, you can also look for it by name:
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```shell
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kubectl get deployment --namespace=kube-system
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```
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and look for a deployment named `coredns` or `kube-dns`.
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Your scale target is
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Deployment/<your-deployment-name>
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where `<your-deployment-name>` is the name of your DNS Deployment. For example, if
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the name of your Deployment for DNS is coredns, your scale target is Deployment/coredns.
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{{< note >}}
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CoreDNS is the default DNS service for Kubernetes. CoreDNS sets the label
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`k8s-app=kube-dns` so that it can work in clusters that originally used
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kube-dns.
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{{< /note >}}
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## Enable DNS horizontal autoscaling {#enablng-dns-horizontal-autoscaling}
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In this section, you create a new 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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{{< codenew file="admin/dns/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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```shell
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kubectl apply -f dns-horizontal-autoscaler.yaml
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```
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The output of a successful command is:
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deployment.apps/dns-autoscaler created
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DNS horizontal autoscaling is now enabled.
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## Tune DNS autoscaling parameters {#tuning-autoscaling-parameters}
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Verify that the dns-autoscaler {{< glossary_tooltip text="ConfigMap" term_id="configmap" >}} exists:
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```shell
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kubectl get configmap --namespace=kube-system
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```
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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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dns-autoscaler 1 ...
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...
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Modify the data in the ConfigMap:
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```shell
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kubectl edit configmap dns-autoscaler --namespace=kube-system
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```
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Look for this line:
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```yaml
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linear: '{"coresPerReplica":256,"min":1,"nodesPerReplica":16}'
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```
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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 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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floats.
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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 tuning DNS horizontal autoscaling. Which option to
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use depends on different conditions.
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### Option 1: Scale down the dns-autoscaler deployment to 0 replicas
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This option works for all situations. Enter this command:
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```shell
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kubectl scale deployment --replicas=0 dns-autoscaler --namespace=kube-system
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```
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The output is:
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deployment.apps/dns-autoscaler scaled
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Verify that the replica count is zero:
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```shell
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kubectl get rs --namespace=kube-system
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```
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The output displays 0 in the DESIRED and CURRENT columns:
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NAME DESIRED CURRENT READY AGE
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...
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dns-autoscaler-6b59789fc8 0 0 0 ...
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...
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### Option 2: Delete the dns-autoscaler deployment
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This option works if dns-autoscaler is under your own control, which means
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no one will re-create it:
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```shell
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kubectl delete deployment dns-autoscaler --namespace=kube-system
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```
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The output is:
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deployment.apps "dns-autoscaler" deleted
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### Option 3: Delete the dns-autoscaler manifest file from the master node
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This option works if dns-autoscaler is under control of the (deprecated)
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[Addon Manager](https://git.k8s.io/kubernetes/cluster/addons/README.md),
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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 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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dns-autoscaler Deployment.
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<!-- 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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## {{% heading "whatsnext" %}}
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* Read about [Guaranteed Scheduling For Critical Add-On Pods](/docs/tasks/administer-cluster/guaranteed-scheduling-critical-addon-pods/).
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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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