Common Cluster Exceptions of TiDB on Kubernetes

This document describes the common exceptions during the operation of TiDB clusters on Kubernetes and their solutions.

TiKV Store is in Tombstone status abnormally

Normally, when a TiKV Pod is in a healthy state (Running), the corresponding TiKV store is also in a healthy state (UP). However, concurrent scale-in or scale-out on the TiKV component might cause part of TiKV stores to fall into the Tombstone state abnormally. When this happens, try the following steps to fix it:

1. View the state of the TiKV store:

   kubectl get -n ${namespace} tidbcluster ${cluster_name} -ojson | jq '.status.tikv.stores'
   

2. View the state of the TiKV Pod:

   kubectl get -n ${namespace} po -l app.kubernetes.io/component=tikv
   

3. Compare the state of the TiKV store with that of the Pod. If the store corresponding to a TiKV Pod is in the Offline state, it means the store is being taken offline abnormally. You can use the following commands to cancel the offline process and perform recovery operations:

   1. Open the connection to the PD service:

      kubectl port-forward -n ${namespace} svc/${cluster_name}-pd ${local_port}:2379 &>/tmp/portforward-pd.log &
      

   2. Bring online the corresponding store:

      curl -X POST http://127.0.0.1:2379/pd/api/v1/store/${store_id}/state?state=Up
      

4. If the TiKV store with the latest lastHeartbeatTime that corresponds to a Pod is in a Tombstone state, it means that the offline process is completed. At this time, you need to re-create the Pod and bind it with a new PV to perform recovery by taking the following steps:

   1. Set the reclaimPolicy value of the PV corresponding to the store to Delete:

      kubectl patch $(kubectl get pv -l app.kubernetes.io/instance=${release_name},tidb.pingcap.com/store-id=${store_id} -o name) -p '{"spec":{"persistentVolumeReclaimPolicy":"Delete"}}'
      

   2. Remove the PVC used by the Pod:

      kubectl delete -n ${namespace} pvc tikv-${pod_name} --wait=false
      

   3. Remove the Pod, and wait for it to be re-created:

      kubectl delete -n ${namespace} pod ${pod_name}
      

   After the Pod is re-created, a new store is registered in the cluster. Then the recovery is completed.

Persistent connections are abnormally terminated in TiDB

Load balancers often set the idle connection timeout. If no data is sent via a connection for a specific period of time, the load balancer closes the connection.

• If a persistent connection is terminated when you use TiDB, check the middleware program between the client and the TiDB server.
• If the idle timeout is not long enough for your query, try to set the timeout to a larger value. If you cannot reset it, enable the tcp-keep-alive option in TiDB.

In Linux, the keepalive probe packet is sent every 7,200 seconds by default. To shorten the interval, configure sysctls via the podSecurityContext field.

• If --allowed-unsafe-sysctls=net.* can be configured for kubelet in the Kubernetes cluster, configure this parameter for kubelet and configure TiDB in the following way:

  tidb:
    ...
    podSecurityContext:
      sysctls:
      - name: net.ipv4.tcp_keepalive_time
        value: "300"
  

• If --allowed-unsafe-sysctls=net.* cannot be configured for kubelet, configure TiDB in the following way:

  tidb:
    annotations:
      tidb.pingcap.com/sysctl-init: "true"
    podSecurityContext:
      sysctls:
      - name: net.ipv4.tcp_keepalive_time
        value: "300"
    ...