Placement group rules and limitations - Amazon Elastic Compute Cloud
General rules and limitationsCluster placement group rules and limitationsPartition placement group rules and limitationsSpread placement group rules and limitations
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Placement group rules and limitations

General rules and limitations

Before you use placement groups, be aware of the following rules:

  • You can create a maximum of 500 placement groups per account in each Region.

  • The name that you specify for a placement group must be unique within your Amazon account for the Region.

  • You can't merge placement groups.

  • An instance can be launched in one placement group at a time; it cannot span multiple placement groups.

  • On-Demand Capacity Reservations and zonal Reserved Instances allow you to reserve capacity for EC2 instances in Availability Zones. When you launch an instance, if the instance attributes match those specified by an On-Demand Capacity Reservation or a zonal Reserved Instance, then the reserved capacity is automatically used by the instance. This is also true if you launch the instance into a placement group.

    If you plan to launch instances into a cluster placement group, we recommend that you reserve capacity explicitly in the cluster placement group. You can do this by creating an On-Demand Capacity Reservation in a specified cluster placement group. Note that while you can reserve capacity in this way using On-Demand Capacity Reservations, the same can't be done with zonal Reserved Instances as they can't reserve capacity explicitly in a placement group.

  • You can't launch Dedicated Hosts in placement groups.

  • You can't launch a Spot Instance that is configured to stop or hibernate on interruption in a placement group.

Cluster placement group rules and limitations

The following rules apply to cluster placement groups:

  • The following instance types are supported:

    • Current generation instances, except for burstable performance instances (for example, T2) and M7i-flex instances.

    • The following previous generation instances: A1, C3, C4, I2, M4, R3, and R4.

  • A cluster placement group can't span multiple Availability Zones.

  • The maximum network throughput speed of traffic between two instances in a cluster placement group is limited by the slower of the two instances. For applications with high-throughput requirements, choose an instance type with network connectivity that meets your requirements.

  • For instances that are enabled for enhanced networking, the following rules apply:

    • Instances within a cluster placement group can use up to 10 Gbps for single-flow traffic. Instances that are not within a cluster placement group can use up to 5 Gbps for single-flow traffic.

    • Traffic to and from Amazon S3 buckets within the same Region over the public IP address space or through a VPC endpoint can use all available instance aggregate bandwidth.

  • 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.

  • Network traffic to the internet and over an Amazon Direct Connect connection to on-premises resources is limited to 5 Gbps for cluster placement groups.

Partition placement group rules and limitations

The following rules apply to partition placement groups:

  • A partition placement group supports a maximum of seven partitions per Availability Zone. The number of instances that you can launch in a partition placement group is limited only by your account limits.

  • When instances are launched into a partition placement group, Amazon EC2 tries to evenly distribute the instances across all partitions. Amazon EC2 doesn’t guarantee an even distribution of instances across all partitions.

  • A partition placement group with Dedicated Instances can have a maximum of two partitions.

  • Capacity Reservations do not reserve capacity in a partition placement group.

Spread placement group rules and limitations

The following rules apply to spread placement groups:

  • A rack spread placement group supports a maximum of seven running instances per Availability Zone. For example, in a Region with three Availability Zones, you can run a total of 21 instances in the group, with seven instances in each Availability Zone. If you try to start an eighth instance in the same Availability Zone and in the same spread placement group, the instance will not launch. If you need more than seven instances in an Availability Zone, we recommend that you use multiple spread placement groups. Using multiple spread placement groups does not provide guarantees about the spread of instances between groups, but it does help ensure the spread for each group, thus limiting the impact from certain classes of failures.

  • Spread placement groups are not supported for Dedicated Instances.

  • Host level spread placement groups are only supported for placement groups on Amazon Outposts. A host level spread placement group can hold as many instances as you have hosts in your Outpost deployment.

  • In a Region, a rack level spread placement group can have a maximum of seven running instances per Availability Zone per group. With Amazon Outposts, a rack level spread placement group can hold as many instances as you have racks in your Outpost deployment.

  • Capacity Reservations do not reserve capacity in a spread placement group.