Amazon Elastic Compute Cloud
User Guide for Linux Instances

Placement Groups

When you launch a new EC2 instance, the EC2 service attempts to place the instance in such a way that all of your instances are spread out across underlying hardware to minimize correlated failures. You can use placement groups to influence the placement of a group of interdependent instances to meet the needs of your workload. Depending on the type of workload, you can create a placement group using one of the following placement strategies:

  • Cluster – packs instances close together inside an Availability Zone. This strategy enables workloads to achieve the low-latency network performance necessary for tightly-coupled node-to-node communication that is typical of HPC applications.

  • Partition – spreads your instances across logical partitions such that groups of instances in one partition do not share the underlying hardware with groups of instances in different partitions. This strategy is typically used by large distributed and replicated workloads, such as Hadoop, Cassandra, and Kafka.

  • Spread – strictly places a small group of instances across distinct underlying hardware to reduce correlated failures.

There is no charge for creating a placement group.

Cluster Placement Groups

A cluster placement group is a logical grouping of instances within a single Availability Zone. A placement group can span peered VPCs in the same Region. The chief benefit of a cluster placement group, in addition to a 10 Gbps flow limit, is the non-blocking, non-oversubscribed, fully bi-sectional nature of the connectivity. In other words, all nodes within the placement group can talk to all other nodes within the placement group at the full line rate of 10 Gbps flows and 100 Gbps aggregate without any slowing due to over-subscription.

The following image shows instances that are placed into a cluster placement group.


                A cluster placement group

Cluster placement groups are recommended for applications that benefit from low network latency, high network throughput, or both, and if the majority of the network traffic is between the instances in the group. To provide the lowest latency and the highest packet-per-second network performance for your placement group, choose an instance type that supports enhanced networking. For more information, see Enhanced Networking.

We recommend that you launch the number of instances that you need in the placement group in a single launch request and that you use the same instance type for all instances in the placement group. If you try to add more instances to the placement group later, or if you try to launch more than one instance type in the placement group, you increase your chances of getting an insufficient capacity error.

If you stop an instance in a placement group and then start it again, it still runs in the placement group. However, the start fails if there isn't enough capacity for the instance.

If you receive a capacity error when launching an instance in a placement group that already has running instances, stop and start all of the instances in the placement group, and try the launch again. Restarting the instances may migrate them to hardware that has capacity for all the requested instances.

Partition Placement Groups

Partition placement groups help reduce the likelihood of correlated hardware failures for your application. When using partition placement groups, Amazon EC2 divides each group into logical segments called partitions. Amazon EC2 ensures that each partition within a placement group has its own set of racks. Each rack has its own network and power source. No two partitions within a placement group share the same racks, allowing you to isolate the impact of hardware failure within your application.

The following image is a simple visual representation of a partition placement group in a single Availability Zone. It shows instances that are placed into a partition placement group with three partitions—Partition 1, Partition 2, and Partition 3. Each partition comprises multiple instances. The instances in a partition do not share racks with the instances in the other partitions, allowing you to contain the impact of a single hardware failure to only the associated partition.


                A partition placement group with three partitions

Partition placement groups can be used to deploy large distributed and replicated workloads, such as HDFS, HBase, and Cassandra, across distinct racks. When you launch instances into a partition placement group, Amazon EC2 tries to distribute the instances evenly across the number of partitions that you specify. You can also launch instances into a specific partition to have more control over where the instances are placed.

A partition placement group can have partitions in multiple Availability Zones in the same Region. A partition placement group can have a maximum of seven partitions per Availability Zone. The number of instances that can be launched into a partition placement group is limited only by the limits of your account.

In addition, partition placement groups offer visibility into the partitions—you can see which instances are in which partitions. You can share this information with topology-aware applications, such as HDFS, HBase, and Cassandra. These applications use this information to make intelligent data replication decisions for increasing data availability and durability.

If you start or launch an instance in a partition placement group and there is insufficient unique hardware to fulfill the request, the request fails. Amazon EC2 makes more distinct hardware available over time, so you can try your request again later.

Spread Placement Groups

A spread placement group is a group of instances that are each placed on distinct racks, with each rack having its own network and power source.

The following image shows seven instances in a single Availability Zone that are placed into a spread placement group. The seven instances are placed on seven different racks.


                A spread placement group

Spread placement groups are recommended for applications that have a small number of critical instances that should be kept separate from each other. Launching instances in a spread placement group reduces the risk of simultaneous failures that might occur when instances share the same racks. Spread placement groups provide access to distinct racks, and are therefore suitable for mixing instance types or launching instances over time.

A spread placement group can span multiple Availability Zones in the same Region. You can have a maximum of seven running instances per Availability Zone per group.

If you start or launch an instance in a spread placement group and there is insufficient unique hardware to fulfill the request, the request fails. Amazon EC2 makes more distinct hardware available over time, so you can try your request again later.

Placement Group Rules and Limitations

General Rules and Limitations

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

  • The name you specify for a placement group must be unique within your AWS 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 Reservation and zonal Reserved Instances provide a capacity reservation for EC2 instances in a specific Availability Zone. The capacity reservation can be used by instances in a placement group. However, it is not possible to explicitly reserve capacity for a placement group.

  • Instances with a tenancy of host cannot be launched in placement groups.

Cluster Placement Group Rules and Limitations

The following rules apply to cluster placement groups:

  • The following are the only instance types that you can use when you launch an instance into a cluster placement group:

    • General purpose: A1, M4, M5, M5a, M5ad, M5d, M5dn, and M5n

    • Compute optimized: C3, C4, C5, C5d, C5n, and cc2.8xlarge

    • Memory optimized: cr1.8xlarge, R3, R4, R5, R5a, R5ad, R5d, R5dn, R5n, X1, X1e, and z1d

    • Storage optimized: D2, H1, hs1.8xlarge, I2, I3, and I3en

    • Accelerated computing: F1, G2, G3, G4dn, P2, P3, and P3dn

  • 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 AWS Direct Connect connection to on-premises resources is limited to 5 Gbps.

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.

  • Partition placement groups are not supported for Dedicated Hosts.

Spread Placement Group Rules and Limitations

The following rules apply to spread placement groups:

  • A 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 (seven per 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 to have more than seven instances in an Availability Zone, then the recommendation is to use multiple spread placement groups. This does not provide guarantees about the spread of instances between groups, but does ensure the spread for each group to limit impact from certain classes of failures.

  • Spread placement groups are not supported for Dedicated Instances or Dedicated Hosts.

Creating a Placement Group

You can create a placement group using the Amazon EC2 console or the command line.

To create a placement group (console)

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

  2. In the navigation pane, choose Placement Groups, Create Placement Group.

  3. Specify a name for the group.

  4. Choose the strategy for the group. If you choose Partition, specify the number of partitions within the group.

  5. Choose Create.

To create a placement group (command line)

To create a partition placement group (AWS CLI)

  • Use the create-placement-group command and specify the --strategy parameter with the value partition and the --partition-count parameter. In this example, the partition placement group is named HDFS-Group-A and is created with five partitions.

    aws ec2 create-placement-group --group-name HDFS-Group-A --strategy partition --partition-count 5

Launching Instances in a Placement Group

You can create an AMI specifically for the instances to be launched in a placement group. To do this, launch an instance and install the required software and applications on the instance. Then, create an AMI from the instance. For more information, see Creating an Amazon EBS-Backed Linux AMI.

To launch instances into a placement group (console)

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

  2. In the navigation pane, choose Instances.

  3. Choose Launch Instance. Complete the wizard as directed, taking care to do the following:

    • On the Choose an Amazon Machine Image (AMI) page, select an AMI. To select an AMI you created, choose My AMIs.

    • On the Choose an Instance Type page, select an instance type that can be launched into a placement group.

    • On the Configure Instance Details page, the following fields are applicable to placement groups:

      • For Number of instances, enter the total number of instances that you need in this placement group, because you might not be able to add instances to the placement group later.

      • For Placement group, select the Add instance to placement group check box. If you do not see Placement group on this page, verify that you have selected an instance type that can be launched into a placement group; otherwise, this option is not available.

      • For Placement group name, you can choose to add the instances to an existing placement group or to a new placement group that you create.

      • For Placement group strategy, choose the appropriate strategy. If you choose partition, for Target partition, choose Auto distribution to have Amazon EC2 do a best effort to distribute the instances evenly across all the partitions in the group; or, specify the partition in which to launch the instances.

To launch instances into a placement group (command line)

  1. Create an AMI for your instances using one of the following commands:

  2. Launch instances into your placement group using one of the following options:

To launch instances into a specific partition of a partition placement group (AWS CLI)

  • Use the run-instances command and specify the placement group name and partition using the --placement "GroupName = HDFS-Group-A, PartitionNumber = 3" parameter. In this example, the placement group is named HDFS-Group-A and the partition number is 3.

    aws ec2 run-instances --placement "GroupName = HDFS-Group-A, PartitionNumber = 3"

Describing Instances in a Placement Group

You can view the placement information of your instances using the Amazon EC2 console or the command line. The placement group is viewable using the console. The partition number for instances in a partition placement group is currently only viewable using the API or AWS CLI.

To view the placement group of an instance (console)

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

  2. In the navigation pane, choose Instances.

  3. Select the instance and, in the details pane, inspect Placement group. If the instance is not in a placement group, the field is empty. Otherwise, the placement group name is displayed. If the placement group is a partition placement group, inspect Partition number for the partition number for the instance.

To view the partition number for an instance in a partition placement group (AWS CLI)

  • Use the describe-instances command and specify the --instance-id parameter.

    aws ec2 describe-instances --instance-id i-0123a456700123456

    The response contains the placement information, which includes the placement group name and the partition number for the instance.

    "Placement": { "AvailabilityZone": "us-east-1c", "GroupName": "HDFS-Group-A", "PartitionNumber": 3, "Tenancy": "default" }

To filter instances for a specific partition placement group and partition number (AWS CLI)

  • Use the describe-instances command and specify the --filters parameter with the placement-group-name and placement-partition-number filters. In this example, the placement group is named HDFS-Group-A and the partition number is 7.

    aws ec2 describe-instances --filters "Name = placement-group-name, Values = HDFS-Group-A" "Name = placement-partition-number, Values = 7"

    The response lists all the instances that are in the specified partition within the specified placement group. The following is example output showing only the instance ID, instance type, and placement information for the returned instances.

    "Instances": [ { "InstanceId": "i-0a1bc23d4567e8f90", "InstanceType": "r4.large", }, "Placement": { "AvailabilityZone": "us-east-1c", "GroupName": "HDFS-Group-A", "PartitionNumber": 7, "Tenancy": "default" } { "InstanceId": "i-0a9b876cd5d4ef321", "InstanceType": "r4.large", }, "Placement": { "AvailabilityZone": "us-east-1c", "GroupName": "HDFS-Group-A", "PartitionNumber": 7, "Tenancy": "default" } ],

Changing the Placement Group for an Instance

You can move an existing instance to a placement group, move an instance from one placement group to another, or remove an instance from a placement group. Before you begin, the instance must be in the stopped state.

You can change the placement group for an instance using the command line or an AWS SDK.

To move an instance to a placement group (command line)

  1. Stop the instance using one of the following commands:

  2. Use the modify-instance-placement command (AWS CLI) and specify the name of the placement group to which to move the instance.

    aws ec2 modify-instance-placement --instance-id i-0123a456700123456 --group-name MySpreadGroup

    Alternatively, use the Edit-EC2InstancePlacement command (AWS Tools for Windows PowerShell).

  3. Restart the instance using one of the following commands:

To remove an instance from a placement group (command line)

  1. Stop the instance using one of the following commands:

  2. Use the modify-instance-placement command (AWS CLI) and specify an empty string for the group name.

    aws ec2 modify-instance-placement --instance-id i-0123a456700123456 --group-name ""

    Alternatively, use the Edit-EC2InstancePlacement command (AWS Tools for Windows PowerShell).

  3. Restart the instance using one of the following commands:

Deleting a Placement Group

If you need to replace a placement group or no longer need one, you can delete it. Before you can delete your placement group, you must terminate all instances that you launched into the placement group, or move them to another placement group.

To terminate or move instances and delete a placement group (console)

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

  2. In the navigation pane, choose Instances.

  3. Select and terminate all instances in the placement group. You can verify that the instance is in a placement group before you terminate it by checking the value of Placement Group in the details pane.

    Alternatively, follow the steps in Changing the Placement Group for an Instance to move the instances to a different placement group.

  4. In the navigation pane, choose Placement Groups.

  5. Select the placement group and choose Delete Placement Group.

  6. When prompted for confirmation, choose Delete.

To terminate instances and delete a placement group (command line)

You can use one of the following sets of commands. For more information about these command line interfaces, see Accessing Amazon EC2.