Schedule Replicas by Topology Labels

To improve the high availability and disaster recovery capability of TiDB clusters, it is recommended that TiKV nodes are physically scattered as much as possible. For example, TiKV nodes can be distributed on different racks or even in different data centers. According to the topology information of TiKV, the PD scheduler automatically performs scheduling at the background to isolate each replica of a Region as much as possible, which maximizes the capability of disaster recovery.

To make this mechanism effective, you need to properly configure TiKV and PD so that the topology information of the cluster, especially the TiKV location information, is reported to PD during deployment. Before you begin, see Deploy TiDB Using TiUP first.

Configure labels based on the cluster topology

Configure labels for TiKV

You can use the command-line flag or set the TiKV configuration file to bind some attributes in the form of key-value pairs. These attributes are called labels. After TiKV is started, it reports its labels to PD so users can identify the location of TiKV nodes.

Assume that the topology has three layers: zone > rack > host, and you can use these labels (zone, rack, host) to set the TiKV location in one of the following methods:

• Use the command-line flag:

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  tikv-server --labels zone=<zone>,rack=<rack>,host=<host>
  

• Configure in the TiKV configuration file:

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  [server]
  labels = "zone=<zone>,rack=<rack>,host=<host>"
  

Configure location-labels for PD

According to the description above, the label can be any key-value pair used to describe TiKV attributes. But PD cannot identify the location-related labels and the layer relationship of these labels. Therefore, you need to make the following configuration for PD to understand the TiKV node topology.

• If the PD cluster is not initialized, configure location-labels in the PD configuration file:

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  [replication]
  location-labels = ["zone", "rack", "host"]
  

• If the PD cluster is already initialized, use the pd-ctl tool to make online changes:

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  pd-ctl config set location-labels zone,rack,host
  

The location-labels configuration is an array of strings, and each item corresponds to the key of TiKV labels. The sequence of each key represents the layer relationship of different labels.

Configure isolation-level for PD

If location-labels has been configured, you can further enhance the topological isolation requirements on TiKV clusters by configuring isolation-level in the PD configuration file.

Assume that you have made a three-layer cluster topology by configuring location-labels according to the instructions above: zone -> rack -> host, you can configure the isolation-level to zone as follows:

[replication]
isolation-level = "zone"

If the PD cluster is already initialized, you need to use the pd-ctl tool to make online changes:

pd-ctl config set isolation-level zone

The location-level configuration is an array of strings, which needs to correspond to a key of location-labels. This parameter limits the minimum and mandatory isolation level requirements on TiKV topology clusters.

Configure a cluster using TiUP (recommended)

When using TiUP to deploy a cluster, you can configure the TiKV location in the initialization configuration file. TiUP will generate the corresponding TiKV and PD configuration files during deployment.

In the following example, a two-layer topology of zone/host is defined. The TiKV nodes of the cluster are distributed among three zones, each zone with two hosts. In z1, two TiKV instances are deployed per host. In z2 and z3, one TiKV instance is deployed per host. In the following example, tikv-n represents the IP address of the nth TiKV node.

server_configs:
  pd:
    replication.location-labels: ["zone", "host"]

tikv_servers:
# z1
  - host: tikv-1
    config:
      server.labels:
        zone: z1
        host: h1
   - host: tikv-2
    config:
      server.labels:
        zone: z1
        host: h1
  - host: tikv-3
    config:
      server.labels:
        zone: z1
        host: h2
  - host: tikv-4
    config:
      server.labels:
        zone: z1
        host: h2
# z2
  - host: tikv-5
    config:
      server.labels:
        zone: z2
        host: h1
   - host: tikv-6
    config:
      server.labels:
        zone: z2
        host: h2
# z3
  - host: tikv-7
    config:
      server.labels:
        zone: z3
        host: h1
  - host: tikv-8
    config:
      server.labels:
        zone: z3
        host: h2s

For details, see Geo-distributed Deployment topology.

PD schedules based on topology label

PD schedules replicas according to the label layer to make sure that different replicas of the same data are scattered as much as possible.

Take the topology in the previous section as an example.

Assume that the number of cluster replicas is 3 (max-replicas=3). Because there are 3 zones in total, PD ensures that the 3 replicas of each Region are respectively placed in z1, z2, and z3. In this way, the TiDB cluster is still available when one data center fails.

Then, assume that the number of cluster replicas is 5 (max-replicas=5). Because there are only 3 zones in total, PD cannot guarantee the isolation of each replica at the zone level. In this situation, the PD scheduler will ensure replica isolation at the host level. In other words, multiple replicas of a Region might be distributed in the same zone but not on the same host.

In the case of the 5-replica configuration, if z3 fails or is isolated as a whole, and cannot be recovered after a period of time (controlled by max-store-down-time), PD will make up the 5 replicas through scheduling. At this time, only 4 hosts are available. This means that host-level isolation cannot be guaranteed and that multiple replicas might be scheduled to the same host. But if the isolation-level value is set to zone instead of being left empty, this specifies the minimum physical isolation requirements for Region replicas. That is to say, PD will ensure that replicas of the same Region are scattered among different zones. PD will not perform corresponding scheduling even if following this isolation restriction does not meet the requirement of max-replicas for multiple replicas.

For example, a TiKV cluster is distributed across three data zones z1, z2, and z3. Each Region has three replicas as required, and PD distributes the three replicas of the same Region to these three data zones respectively. If a power outage occurs in z1 and cannot be recovered after a period of time, PD determines that the Region replicas on z1 are no longer available. However, because isolation-level is set to zone, PD needs to strictly guarantee that different replicas of the same Region will not be scheduled on the same data zone. Because both z2 and z3 already have replicas, PD will not perform any scheduling under the minimum isolation level restriction of isolation-level, even if there are only two replicas at this moment.

Similarly, when isolation-level is set to rack, the minimum isolation level applies to different racks in the same data center. With this configuration, the isolation at the zone layer is guaranteed first if possible. When the isolation at the zone level cannot be guaranteed, PD tries to avoid scheduling different replicas to the same rack in the same zone. The scheduling works similarly when isolation-level is set to host where PD first guarantees the isolation level of rack, and then the level of host.

In summary, PD maximizes the disaster recovery of the cluster according to the current topology. Therefore, if you want to achieve a certain level of disaster recovery, deploy more machines on different sites according to the topology than the number of max-replicas. TiDB also provides mandatory configuration items such as isolation-level for you to more flexibly control the topological isolation level of data according to different scenarios.