Amazon EC2 Dedicated Host auto-placement and host affinity

Placement control for Dedicated Hosts happens on both the instance level and host level.

Auto-placement

Auto-placement is configured at the host level. It allows you to manage whether instances that you launch are launched onto a specific host, or onto any available host that has matching configurations.

When auto-placement is disabled for a Dedicated Host, it accepts only host tenancy instance launches that specify its unique host ID. This is the default setting for new Dedicated Hosts.

When auto-placement is enabled for a Dedicated Host, it accepts any untargeted, host tenancy instance launches that match its instance type configuration.

When launching an instance, you need to configure its tenancy. Launching an instance onto a Dedicated Host without providing a specific HostId enables it to launch on any Dedicated Host that has auto-placement enabled and that matches its instance type.

Host affinity

Host affinity is configured at the instance level. It establishes a launch relationship between an instance and a Dedicated Host.

When affinity is set to Host, an instance launched onto a specific host always restarts on the same host if stopped. This applies to both targeted and untargeted launches.

When affinity is set to Default, and you stop and restart the instance, it can be restarted on any available host. However, it tries to launch back onto the last Dedicated Host on which it ran (on a best-effort basis).