When provisioning a PostgreSQL HA Instance using NDB, the minimum and maximum number of DB server VMs that can be deployed in a cluster are 2 and 4, respectively. A PostgreSQL HA Instance is a database instance that provides high availability and fault tolerance for PostgreSQL databases using the Patroni framework. Patroni is a Python-based tool that manages PostgreSQL configuration and performs automatic failover and switchover operations. Patroni relies on a distributed consensus store, such as etcd, Consul, or ZooKeeper, to store and synchronize the cluster state and leader information. NDB supports etcd as the consensus store for PostgreSQL HA Instances. NDB allows the administrator to provision PostgreSQL HA Instances with different sizes and configurations, such as small, medium, large, or custom. The size of the PostgreSQL HA Instance determines the number of DB server VMs, the number of etcd nodes, the CPU, memory, and disk resources, and the replication factor of the database. The minimum number of DB server VMs for a PostgreSQL HA Instance is 2, which corresponds to a small size instance. This configuration provides one leader and one follower DB server VM, and one etcd node. The maximum number of DB server VMs for a PostgreSQL HA Instance is 4, which corresponds to a large size instance. This configuration provides one leader and three follower DB server VMs, and three etcd nodes. The other options are not valid for a PostgreSQL HA Instance, as they either do not provide enough redundancy or exceed the supported limit of DB server VMs. References:
Nutanix Certified Professional - Database Automation (NCP-DB) v6.5, Section 2 - Deploy and Configure an NDB Solution, Objective 2.2: Configure an NDB Instance
Nutanix Database Management & Automation (NDMA) Course, Module 3: Nutanix Database Service (NDB) Installation and Configuration, Lesson 3.2: Configuring NDB, Topic: Provisioning PostgreSQL HA Instances
Nutanix Database Service (NDB) User Guide, Chapter 4: Provisioning Databases, Section: Provisioning PostgreSQL HA Instances
[PostgreSQL High Availability: Under the Hood – Nutanix.dev], Section: PostgreSQL HA Architecture and Components
