Overview

An affinity group is a simple orchestration policy designed for IaaS resources to ensure your business high performances or high availability.

Affinity Group Policy

Currently, ZStack provides two affinity group policies to better manage VM instances and hosts: anti-affinity group (soft) and anti-affinity group (hard).
  • Anti-affinity group (soft):

    Allocate VM instances in the affinity group to different hosts as much as possible. If no more hosts are available, the VM instances will be allocated randomly.

  • Anti-affinity group (hard):

    Strictly allocate VM instances in the affinity group to different hosts. If no more hosts are available, the allocation fails.

The logic of these two anti-affinity groups is shown in Figure 1.
Figure 1. Anti-Affinity Group (Soft) and Anti-Affinity Group (Hard)


Usage Scenario

This section provides some usage examples of anti-affinity group (soft) and anti-affinity group (hard) policies.

  • Anti-affinity group (soft):
    You might want to deploy nodes with different Hadoop roles on different hosts to improve the overall system performance.
    • For example, when you deploy a Hadoop system, you might find it difficult to calculate the exact number of nodes of different roles such as NameNode, DataNode, JobTracker, and TaskTracker. However, you might know that deploying these nodes on different hosts is more effective. With the anti-affinity group (soft) policy, you can deploy Hadoop clusters on different hosts as much as possible, which relieves the I/O pressure and improves the overall performance of the system.
  • Anti-affinity group (hard):
    You might want to deploy two VM instances that run an active and a standby database on different hosts to ensure high availability.
    • For example, you deploy two business VM instances to run an active and a standby MySQL database respectively, and requires that the active and standby databases cannot be down at the same time. Therefore, you must deploy these two VM instances on different hosts. Due to deployment automation, you might not predict which hosts have sufficient resources. With the anti-affinity group (hard) policy, you can choose two different hosts to run these two VM instances respectively, which ensures the high availability.
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