Planning Cluster AZs and HA

An availability zone (AZ) is a physical region where resources use independent power supplies and networks. AZs in the same region can communicate with each other through internal networks but are physically isolated.

Multi-AZ deployment is a high availability feature provided by CSS. Deploying a cluster across two AZs located in the same region can help prevent data loss and lower the possibility of service outages.

If you select multi-AZ deployment when creating a cluster, CSS automatically enables cross-AZ HA to ensure that cluster nodes will be evenly distributed across the selected AZs (the difference between the number of nodes in each AZ cannot exceed 1).

Node Distribution Rules

When a multi-AZ cluster is created, nodes of all types are evenly distributed across different AZs. A maximum of two AZs can be used. Table 1 shows the node distribution when the number of AZs varies.

When creating a multi-AZ cluster, select at least two nodes. Otherwise, multi-AZ cluster deployment will fail.

Table 1 Node distribution when the numbers of nodes and AZs vary

Node Quantity	Single AZ	Two AZs
	AZ1	AZ1	AZ2
1	1	Not supported
2	2	1	1
3	3	2	1
4	4	2	2
...	...	...	...