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Overview

HA Policy is a mechanism that ensures sustained and stable running of the business if VM instances are unexpectedly or scheduled stopped or are errored because of errors occurring to compute, network, or storage resources associated with the VM instances. By enabling this feature, you can customize VM HA policies to ensure your business continuity and stability.

Concepts

The HA Policy feature involves the following key concepts:

HA mode: Specifies whether to enable auto restart if VM instances are unexpectedly or scheduled stopped or are errored because of errors occurring to compute, network, or storage resources associated with the VM instances. None and NeverStop are supported:
- None: VM instances scheduled to be stopped or unexpectedly stopped are not auto restarted.
- NeverStop:
  - VM instances scheduled to be stopped are auto restarted.
  - Unexpectedly stopped VM instances are auto restarted on another host depending on the failover strategy you configure for them.

VM Failover Strategy: Specifies whether to migrate a VM instance to another host if errors occur to the compute resource, storage resource, or network resource associated with the VM instance.

The VM failover mechanism inspects the following resource status:

Management Network Connectivity Status:
- Management network connectivity status indicates the status of the network that connects the management node and the host where VM instances reside.
- This status may turn Abnormal if errors occur to the management node or to the management network.

Storage Network Connectivity Status:
- Detects the connectivity status of the network that VM instances use to access the primary storage where the root volumes of these VM instances reside.
- This status may turn Abnormal if errors occur to the primary storage or to the storage network.
Business NIC Status:
- Business NIC status may turn Abnormal if errors occur to the host business NIC or the switch port directly connecting to the host business NIC that is associated with the L2 network of VM instances.

Based on the resource status inspection, the Cloud provides the following truth table for configuring VM failover strategies:

Management Network Connectivity Status	Storage Network Connectivity Status	Business NIC Status	Fail Over
Normal	Normal	Abnormal	Yes/No
Normal	Abnormal	Normal	Yes/No
Normal	Abnormal	Abnormal	Yes/No
Abnormal	Normal	Normal	No

Fundamentals

ZStack Cloud HA Policy has the following mechanisms：

The Cloud polls the running status of VM instances. If a VM instance is scheduled or unexpectedly stopped, its HA mode is checked. If the HA mode of the VM instance is NeverStop, then the VM instance is restarted on the current host or another host.

Figure 1. VM HA Started After Unexpectedly Stopped
The Cloud polls the status of the hosts where VM instances reside. Either of the management network connectivity status, storage network connectivity status, and business NIC status of the host turns abnormal, the corresponding VM failover strategy and VM HA mode are checked. If the corresponding failover strategy is Yes and VM HA mode is NeverStop, then related VM instances are migrated to another host.

Figure 2. VM HA Started After Host Business NIC Turns Down

Characteristics

HA Policy has the following characteristics:

Comprehensive & Powerful: Covers all mainstream HA scenarios, including various failures, and ensures the stability and continuity of your business.
Flexible & Visualized: Provides a simple table that allows you to configure VM failover strategies with one click. This table functions together with the HA Mode that can be configured on all and individual VM instances, thus greatly improving the flexibility of your business HA configuration.

Scenarios

The following describes the scenarios of the HA Policy feature.

Host Business NIC Turns Down:
If a host business NIC turns down, to ensure high availability of business, all VM instances associated with this NIC are expected to migrate to other hosts.
- For example, your business VM instances are running MySQL database service which is required to achieve high availability. In this case, you can set the HA mode of these VM instances to NeverStop and turn on the switch corresponding to Abnormal Business NIC Status. Then as long as host resources are sufficient, in case that a host business NIC associated with these VM instances turns down, these VM instances will be auto started on other hosts.
VM Unexpectedly Stops:
If a VM instance is unexpectedly stopped, it is expected to auto HA start.
- For example, your VM instances are running important business applications. To ensure business auto-recovery in case of VM stops due to reasons such as host powered-offs or business overloads, you can set the HA mode of these VM instances to NeverStop. Then if these VM instances are stopped, they are auto started.

Next Chapter: Preparations

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