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Rules and limitations
You can launch multiple instance types into a cluster placement group. However, this reduces the likelihood that the required capacity will be available for your launch to succeed. We recommend using the same instance type for all instances in a cluster placement group.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-strategies.html
There are no restrictions on instance types within a placement group, so you can launch instances of different families in the same placement group. You are not limited to a single instance family like m6g.
Regarding the number of instances a placement group can contain, there is no hard limit for cluster placement groups. However, it's important to note that adding more instances to a cluster placement group or trying to launch multiple instance types in the group increases the chances of getting an insufficient capacity error.
For other placement group types, there are some limitations:
- Partition placement groups can have a maximum of seven partitions per Availability Zone.
- Spread placement groups at the rack level can have a maximum of seven running instances per Availability Zone.
It's also worth noting that there is an initial limit of 500 placement groups that can be created per AWS region, but this can be increased by contacting AWS support.
When launching instances in a placement group, it's advisable to use a single launch request and the same instance type for all instances to reduce the likelihood of capacity errors. If you stop an instance in a placement group and then start it again, it will still run in the placement group, but the start may fail if there isn't enough capacity for the instance.
Sources
Placement strategies for your placement groups - Amazon Elastic Compute Cloud
Amazon EMR integration with EC2 placement groups - Amazon EMR
Placement groups for your Amazon EC2 instances - Amazon Elastic Compute Cloud
Thank you, Gary - Are there any SLAs on latency between nodes in a placement group?
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Thank you, Gary - Are there any SLAs on latency between nodes in a placement group?