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Scale TiDB in Kubernetes
This document introduces how to horizontally and vertically scale a TiDB cluster in Kubernetes.
Horizontally scaling TiDB means that you scale TiDB out or in by adding or remove nodes 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. Scaling out operations add nodes based on the node ID in ascending order, while scaling in operations remove nodes based on the node ID in descending order.
Horizontal scaling operations
To perform a horizontal scaling operation:
Modify pd.replicas, tidb.replicas, tikv.replicas in the value.yaml file of the cluster to a desired value.
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.
The PD and TiKV components do not trigger scaling in and out operations during the rolling update.
When the TiKV component scales in, it 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 tidbcluster -n <namespace> <release-name> -o json | jq '.status.tikv.stores'.
When the PD and TiKV 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.
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.
Vertically scaling TiDB means that you scale TiDB up or down by increasing or decreasing the limit of resources on the node. Vertically scaling is essentially the rolling update of the nodes.
Vertical scaling operations
To perform a vertical scaling operation:
Modify tidb.resources, tikv.resources, pd.resources in the values.yaml file to a desired value.
When all Pods are rebuilt and in the Running state, the vertical scaling is completed.
If the resource's requests field is modified during the vertical scaling process, because PD and TiKV use Local PV, they need to 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.