Placement group is a policy that determines how instances are distributed and placed on the underlying hardware. Instances that you create in a placement group are launched with disaster recovery and high availability.
The CVM provides an instance placement policy, which can forcibly break up instances according to a certain policy when they are created, so as to reduce the impact of underlying hardware/software failures on the business on the CVM. You can use placement groups to deploy CVM instances involved in your business on different physical servers to ensure high availability and underlying disaster recovery capabilities of your business.
When creating instances within a placement group, we will spin up the instances in a dispersed manner within the specified region, based on the deployment policy you have previously set.
If you do not set a placement group for your instance, we will try to launch the instance on different physical machines to ensure service availability.
Distributed Placement Groups
Currently, placement groups support distributed groups. A distributed placement group is a group of instances placed on different underlying hardware to provide high availability. It is recommended that applications that require separate placement of important instances use distributed placement groups, such as primary-secondary databases, high-availability clusters, and other applications. By launching instances in distributed placement groups, you reduce the risk of instances with the same underlying hardware failing at the same time.
Distributed placement groups have regional attributes and can span multiple availability zones. There is a limit on the number of instances per group. For more information, see the console page.
If there is insufficient hardware to fulfill the request, the request will fail when launching instances in a distributed placement group. Generally, you can wait for a while and try your operation again.
Rules and Limitations of Distributed Placement Groups
Before using distributed placement groups, you should be aware of the following rules:
- Placement groups cannot be merged.
- Instances cannot span multiple placement groups.
- You can choose to distribute the content at three levels: physical machine, switch, and rack.
For example, if you select the physical machine level, the instances of the distributed group will not be placed on the same physical machine to ensure the high availability of the business. - The maximum number of instances supported by groups at different placement levels is different. The specific value is subject to the console page.
- After using the disaster recovery group policy, the policy you specify will be strictly followed. It is important to note that if the underlying hardware is not sufficient to distribute the instances, some instances will fail to be created.
- Instances on CDHs do not support distributed placement groups.