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.
You can use the command-line flag or set the TiKV or TiFlash configuration file to bind some attributes in the form of key-value pairs. These attributes are called
labels. After TiKV and TiFlash are started, they report their
labels to PD so users can identify the location of TiKV and TiFlash nodes.
Assume that the topology has four layers: zone > data center (dc) > rack > host, and you can use these labels (zone, dc, rack, host) to set location of the TiKV and TiFlash. To set labels for TiKV and TiFlash, you can use one of the following methods:
Use the command-line flag to start a TiKV instance:tikv-server --labels zone=<zone>,dc=<dc>,rack=<rack>,host=<host>
Configure in the TiKV configuration file:[server] [server.labels] zone = "<zone>" dc = "<dc>" rack = "<rack>" host = "<host>"
To set labels for TiFlash, you can use the
tiflash-learner.toml file, which is the configuration file of tiflash-proxy:
[server] [server.labels] zone = "<zone>" dc = "<dc>" rack = "<rack>" host = "<host>"
When Follower read is enabled, if you want TiDB to prefer to read data from the same region, you need to configure
labels for TiDB nodes.
You can set
labels for TiDB using the configuration file:
[labels] zone = "<zone>" dc = "<dc>" rack = "<rack>" host = "<host>"
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.
Defined as an array of strings,
location-labels is the configuration for PD. Each item of this configuration corresponds to the key of TiKV
labels. Besides, the sequence of each key represents the layer relationship of different labels (the isolation levels decrease from left to right).
You can customize the value of
location-labels, such as
host, because the configuration does not have default values. Also, this configuration has no restriction in the number of label levels (not mandatory for 3 levels) as long as they match with TiKV server labels.
location-labels, choose one of the following methods according to your cluster situation:
If the PD cluster is not initialized, configure
location-labelsin the PD configuration file:[replication] location-labels = ["zone", "rack", "host"]
If the PD cluster is already initialized, use the pd-ctl tool to make online changes:pd-ctl config set location-labels zone,rack,host
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
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
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.
When using TiUP to deploy a cluster, you can configure the TiKV location in the initialization configuration file. TiUP will generate the corresponding configuration files for TiKV, PD, and TiFlash 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, z1, z2, and z3, with each zone having four hosts, h1, h2, h3, and h4. In z1, four TiKV instances are deployed on two hosts,
tikv-2 on h1, and
tikv-4 on h2. Two TiFlash instances are deployed on the other two hosts,
tiflash-1 on h3 and
tiflash-2 on h4. In z2 and z3, two TiKV instances are deployed on two hosts, and two TiFlash instances are deployed on the other two hosts. In the following example,
tikv-n represents the IP address of the
nth TiKV node, and
tiflash-n represents the IP address of the
nth TiFlash 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 tiflash_servers: # z1 - host: tiflash-1 learner_config: server.labels: zone: z1 host: h3 - host: tiflash-2 learner_config: server.labels: zone: z1 host: h4 # z2 - host: tiflash-3 learner_config: server.labels: zone: z2 host: h3 - host: tiflash-4 learner_config: server.labels: zone: z2 host: h4 # z3 - host: tiflash-5 learner_config: server.labels: zone: z3 host: h3 - host: tiflash-6 learner_config: server.labels: zone: z3 host: h4
For details, see Geo-distributed Deployment topology.
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 (controlled by
max-store-down-time and 30 minutes by default), 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.
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.