Manually Scale TiDB on Kubernetes

This document introduces how to horizontally and vertically scale a TiDB cluster on Kubernetes.

Horizontal scaling

Horizontally scaling TiDB means that you scale TiDB out or in by adding or remove Pods in your pool of resources. When you scale a TiDB cluster, PD, TiKV, and TiDB are scaled out or in sequentially according to the values of their replicas.

To scale out a TiDB cluster, increase the value of replicas of a certain component. The scaling out operations add Pods based on the Pod ID in ascending order, until the number of Pods equals the value of replicas.
To scale in a TiDB cluster, decrease the value of replicas of a certain component. The scaling in operations remove Pods based on the Pod ID in descending order, until the number of Pods equals the value of replicas.

Horizontally scale PD, TiKV, TiDB, and TiProxy

To scale PD, TiKV, TiDB, or TiProxy horizontally, use kubectl to modify spec.pd.replicas, spec.tikv.replicas, spec.tidb.replicas, and spec.tiproxy.replicas in the TidbCluster object of the cluster to desired values.

Modify the replicas value of a component as needed. For example, configure the replicas value of PD to 3:

kubectl patch -n ${namespace} tc ${cluster_name} --type merge --patch '{"spec":{"pd":{"replicas":3}}}'

Check whether your configuration has been updated in the corresponding TiDB cluster on Kubernetes.
```
kubectl get tidbcluster ${cluster_name} -n ${namespace} -oyaml
```
If your configuration is successfully updated, in the TidbCluster CR output by the command above, the values of spec.pd.replicas, spec.tidb.replicas, and spec.tikv.replicas are consistent with the values you have configured.
Check whether the number of TidbCluster Pods has increased or decreased.
```
watch kubectl -n ${namespace} get pod -o wide
```
For the PD and TiDB components, it might take 10-30 seconds to scale in or out.
For the TiKV component, it might take 3-5 minutes to scale in or out because the process involves data migration.

Horizontally scale TiFlash

This section describes how to horizontally scale out or scale in TiFlash if you have deployed TiFlash in the cluster.

Horizontally scale out TiFlash

To scale out TiFlash horizontally, you can modify spec.tiflash.replicas.

For example, configure the replicas value of TiFlash to 3:

kubectl patch -n ${namespace} tc ${cluster_name} --type merge --patch '{"spec":{"tiflash":{"replicas":3}}}'

Horizontally scale in TiFlash

To scale in TiFlash horizontally, perform the following steps:

Expose the PD service by using port-forward:

kubectl port-forward -n ${namespace} svc/${cluster_name}-pd 2379:2379

Open a new terminal tab or window. Check the maximum number (N) of replicas of all data tables with which TiFlash is enabled by running the following command:
```
curl 127.0.0.1:2379/pd/api/v1/config/rules/group/tiflash | grep count
```
In the printed result, the largest value of count is the maximum number (N) of replicas of all data tables.
Go back to the terminal window in Step 1, where port-forward is running. Press Ctrl+C to stop port-forward.
After the scale-in operation, if the number of remaining Pods in TiFlash >= N, skip to Step 6. Otherwise, take the following steps:
1. Refer to Access TiDB and connect to the TiDB service.
2. For all the tables that have more replicas than the remaining Pods in TiFlash, run the following command:
```
alter table <db_name>.<table_name> set tiflash replica ${pod_number};
```
  ${pod_number} indicates the number of remaining Pods in the TiFlash cluster after scaling in.
Wait for the number of TiFlash replicas in the related tables to be updated.
Connect to the TiDB service, and run the following command to check the number:
```
SELECT * FROM information_schema.tiflash_replica WHERE TABLE_SCHEMA = '<db_name>' and TABLE_NAME = '<table_name>';
```
If you cannot view the replication information of related tables, the TiFlash replicas are successfully deleted.
Modify spec.tiflash.replicas to scale in TiFlash.
Check whether TiFlash in the TiDB cluster on Kubernetes has updated to your desired definition. Run the following command and see whether the value of spec.tiflash.replicas returned is expected:
```
kubectl get tidbcluster ${cluster-name} -n ${namespace} -oyaml
```

Horizontally scale TiCDC

If TiCDC is deployed in the cluster, you can horizontally scale out or scale in TiCDC by modifying the value of spec.ticdc.replicas.

For example, configure the replicas value of TiCDC to 3:

kubectl patch -n ${namespace} tc ${cluster_name} --type merge --patch '{"spec":{"ticdc":{"replicas":3}}}'

View the horizontal scaling status

To view the scaling status of the cluster, run the following command:

watch kubectl -n ${namespace} get pod -o wide

When the number of Pods for all components reaches the preset value and all components go to the Running state, the horizontal scaling is completed.

Note

The PD, TiKV and TiFlash components do not trigger the rolling update operations during scaling in and out.
When the TiKV component scales in, TiDB Operator calls the PD interface to mark the corresponding TiKV instance as offline, and then migrates the data on it to other TiKV nodes. During the data migration, the TiKV Pod is still in the Running state, and the corresponding Pod is deleted only after the data migration is completed. The time consumed by scaling in depends on the amount of data on the TiKV instance to be scaled in. You can check whether TiKV is in the Offline state by running kubectl get -n ${namespace} tidbcluster ${cluster_name} -o json | jq '.status.tikv.stores'.
When the number of UP stores is equal to or less than the parameter value of MaxReplicas in the PD configuration, the TiKV components can not be scaled in.
The TiKV component does not support scale out while a scale-in operation is in progress. Forcing a scale-out operation might cause anomalies in the cluster. If an anomaly already happens, refer to TiKV Store is in Tombstone status abnormally to fix it.
The TiFlash component has the same scale-in logic as TiKV.
When the PD, TiKV, and TiFlash components scale in, the PVC of the deleted node is retained during the scaling in process. Because the PV's reclaim policy is changed to Retain, the data can still be retrieved even if the PVC is deleted.

Vertical scaling

Vertically scaling TiDB means that you scale TiDB up or down by increasing or decreasing the limit of resources on the Pod. Vertical scaling is essentially the rolling update of the Pods.

Vertically scale components

This section describes how to vertically scale up or scale down components including PD, TiKV, TiDB, TiProxy, TiFlash, and TiCDC.

To scale up or scale down PD, TiKV, TiDB, and TiProxy, use kubectl to modify spec.pd.resources, spec.tikv.resources, spec.tidb.resources, and spec.tiproxy.replicas in the TidbCluster object that corresponds to the cluster to desired values.
To scale up or scale down TiFlash, modify the value of spec.tiflash.resources.
To scale up or scale down TiCDC, modify the value of spec.ticdc.resources.

View the vertical scaling progress

To view the upgrade progress of the cluster, run the following command:

watch kubectl -n ${namespace} get pod -o wide

When all Pods are rebuilt and in the Running state, the vertical scaling is completed.

Note

If the resource's requests field is modified during the vertical scaling process, and if PD, TiKV, and TiFlash use Local PV, they will be scheduled back to the original node after the upgrade. At this time, if the original node does not have enough resources, the Pod ends up staying in the Pending status and thus impacts the service.
TiDB is a horizontally scalable database, so it is recommended to take advantage of it simply by adding more nodes rather than upgrading hardware resources like you do with a traditional database.

Scale PD microservice components

Note

Starting from v8.0.0, PD supports the microservice mode (experimental).

PD microservices are typically used to address performance bottlenecks in PD and improve the quality of PD services. To determine whether it is necessary to scale PD microservices, see PD microservice FAQs.

Currently, the PD microservices mode splits the timestamp allocation and cluster scheduling functions of PD into two independently deployed components: the tso microservice and the scheduling microservice.
- The tso microservice implements a primary-secondary architecture. If the tso microservice becomes the bottleneck, it is recommended to scale it vertically.
- The scheduling microservice serves as a scheduling component. If the scheduling microservice becomes the bottleneck, it is recommended to scale it horizontally.
To vertically scale each component of PD microservices, use the kubectl command to modify the spec.pdms.resources of the TidbCluster object corresponding to the cluster to your desired value.
To horizontally scale each component of PD microservices, use the kubectl command to modify spec.pdms.replicas of the TidbCluster object corresponding to the cluster to your desired value.

Taking the scheduling microservice as an example, the steps for horizontal scaling are as follows:

Modify the replicas value of the corresponding TidbCluster object to your desired value. For example, run the following command to set the replicas value of scheduling to 3:
```
kubectl patch -n ${namespace} tc ${cluster_name} --type merge --patch '{"spec":{"pdms":[{"name":"scheduling", "replicas":3}]}}'
```
Check whether the corresponding TiDB cluster configuration for the Kubernetes cluster is updated:
```
kubectl get tidbcluster ${cluster_name} -n ${namespace} -oyaml
```
In the output of this command, the scheduling.replicas value of spec.pdms in TidbCluster is expected to be the same as the value you configured.
Observe whether the number of TidbCluster Pods is increased or decreased:
```
watch kubectl -n ${namespace} get pod -o wide
```
It usually takes about 10 to 30 seconds for PD microservice components to scale in or out.

Scaling troubleshooting

During the horizontal or vertical scaling operation, Pods might go to the Pending state because of insufficient resources. See Troubleshoot the Pod in Pending state to resolve it.

Manually Scale TiDB on Kubernetes

Horizontal scaling

Horizontally scale PD, TiKV, TiDB, and TiProxy

Horizontally scale TiFlash

Horizontally scale out TiFlash

Horizontally scale in TiFlash

Horizontally scale TiCDC

View the horizontal scaling status

Vertical scaling

Vertically scale components

View the vertical scaling progress

Scale PD microservice components

Scaling troubleshooting

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