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