AWS Compute Blog
Control instance placement using Asset Level Capacity Management for AWS Outposts
AWS Outposts supports self-service capacity management at the entire Outpost level, or at the individual asset level, making it easy for you to view and manage compute capacity on your Outposts. This feature supports both Outposts rack (such as the recently announced second-generation Outposts rack) and Outposts server. A default capacity configuration for each new Outpost is determined during the ordering process. This default configuration can subsequently be modified to create a range of Amazon Elastic Compute Cloud (Amazon EC2) instance sizes and quantities to meet your changing business needs. For more information on performing Outposts level multi-asset reconfigurations, go to Dynamically reconfigure your AWS Outposts capacity using Capacity Tasks.
The release of Asset Level Capacity Management allows you to control the configuration of specific assets within your Outpost, which can be useful when planning strategies for EC2 Auto Scaling groups and host-level high availability. An Outpost asset can be a single server within an Outposts rack, or an Outposts server. This post focuses on how to use Asset Level Capacity Management to perform single-host reconfigurations, and how this can be used with Amazon EC2 placement groups to control instance placement on your Outpost.
Overview
When you place an Outposts order, you determine the capacity configuration of each Outpost based on the anticipated workload requirements. You can scale your Outposts up or out as needed during your commitment term. For further details on Outpost capacity planning including best practices, refer to the Capacity Planning – AWS Outposts High Availability Design and Architecture whitepaper. We recommend planning spare capacity for N+M host availability per instance family when making modifications to your Outpost capacity configuration for workloads that need to be highly available. To calculate, take the number of assets (N) you need to run all your workloads, and then add (M) additional assets to meet your requirements for server availability during failure and maintenance events.
You also need to plan for instance level high availability when deciding to reconfigure particular assets. For example, say you have two C5 assets, and each one is configured homogeneously to provide C5.2xlarge instances. If you have an Auto Scaling group that specifies C5.2xlarge in its launch template, and you perform an asset level reconfiguration of one of your C5 assets so that it only offers C5.4xlarge instances, then your Auto Scaling group can only launch instances on the one C5 host configured to provide C5.2xlarge instances. If that host fails, then the Auto Scaling group is unable to launch new C5.2xlarge instances on the other host unless the Auto Scaling group launch template is modified. Understanding failure scenario behavior and how much capacity you want to reserve for high availability is key to capacity management and disaster recovery planning. For highly available workloads, we recommend spreading your instances across as many assets as possible.
Understanding EC2 placement groups on AWS Outposts
Outposts rack supports EC2 placement groups, and two placement group options are available only on Outposts: rack level spread, and host level spread. This allows you to spread out instances across underlying hardware on an Outpost at your site. To use a rack level spread placement group, you must have two or more physical Outpost racks. Each spread strategy can be used to create resilient Outposts architectures that can withstand a rack or host failure depending on the respective strategy used.
Rack level spread
Figure 1: Outposts rack showing a rack level spread EC2 placement group
Using a multi-rack Outpost, you can spread your EC2 instances across multiple racks with a rack level spread EC2 placement group. When used with Auto Scaling groups, this allows you to withstand an individual rack or multi-asset failure. When your Auto Scaling group detects you’ve lost instances on one of your racks, it automatically relaunches the instances using the assets on your other racks if you have available capacity. To use this strategy to increase your workload resiliency, each rack would need to have assets that can support the instance type (C5 is used in the preceding figure) and size used in your Auto Scaling group launch template. The expanded functionality that asset level capacity management brings to capacity tasks allows you to configure your Outpost so that each rack has at least one asset that can support the instances used in your Auto Scaling groups. Configure your assets on each rack to meet your resiliency goals for host failure tolerance as well. This configuration can be done in an on-demand, self-service fashion to meet the needs of your evolving workloads if instance requirements change over time.
Host level spread
Figure 2: Outposts rack showing a host level spread EC2 placement group
Although rack level spread EC2 placement groups need a multi-rack Outpost, host level spread EC2 placement groups can be used within a single rack Outpost to provide resiliency for your workloads at the asset level. When used with Auto Scaling groups, this allows you to withstand an individual asset or multi-asset failure depending on your Outpost configuration. When your Auto Scaling group detects you’ve lost instances on one of your assets due to a hardware failure, it automatically relaunches the instances using your other assets on your Outpost if you have available capacity. To use this strategy to increase your workload resiliency, you would need to have at least two assets within your Outposts rack that can support the instance type (R5 and M5 are used in the preceding figure) and size used in your Auto Scaling group launch template. Outposts also supports using attribute-based instance type selection if multiple instance types meet your workload needs based on some minimum resource requirements. With the expanded functionality that asset level capacity management brings to capacity tasks, you can configure your Outposts rack so that each asset type can support the instance size used in your Auto Scaling groups. This configuration can be done in an on-demand, self-service fashion to meet the needs of your evolving workloads if instance requirements change over time.
Using asset level capacity tasks
Asset Level Capacity Management allows you to target a specific Outpost asset to change its capacity configuration directly, allowing granular control over instance capacity pool configurations. Outpost assets are referred to by a unique ten-digit Asset ID. The first step in this process is identifying a suitable asset on which to perform the capacity task. To do this, you can use the rack view within the Outposts console page to view each asset, its current capacity configuration, and its current usage. Choosing an asset with fewer running instances may increase the chances of the capacity task being successful without needing instances to be stopped.
In the following example, the rack view has been filtered by the R5 family resulting in the two R5 assets being displayed. The Show instance details option has also been chosen to show the instance IDs of the running instances on our Outposts rack.
Figure 3: Rack view of the Outposts console
When you have identified the asset to target for the capacity task, you can either choose the Modify option in the top right of the asset itself or go to Capacity Tasks from the console menu and choose the asset ID directly from the dropdown menu.
Figure 4: Capacity tasks console experience
From here, you have the option to use the capacity configuration builder to interactively modify your Outposts capacity layout, or you can upload a capacity configuration plan JSON document with the necessary configuration. When building the capacity task, you have two options to choose from when handling instances that are blocking the task from executing. The default option is set to fail the capacity task if this occurs. However, this can be set to wait for the instances to be stopped so that the task can continue. If this option is chosen, then the asset is placed into an isolated state until either the capacity task completes or is cancelled, thus preventing any further instances launches on the impacted asset.
If there are instances on the asset that can’t be stopped to complete the capacity task, then they can be chosen from the Instances to keep as-is section. Only the instances running on the impacted asset are listed. If a capacity task can’t be completed while leaving the chosen instances running, the capacity task fails.
In the following example, the capacity configuration requested for the asset results in the removal of one r5.4xlarge and two r5.2xlarge instances, which creates sufficient space for the creation of 12 r5.large instances. This asset also has three instances running on it which have all been chosen to keep as-is during the execution of the task.
Figure 5: Capacity task example showing r5 asset level capacity management
You can also execute capacity tasks programmatically If you prefer through CLI or API calls. For example, using the start-capacity-task CLI to submit the same configuration would look as follows:
After defining the capacity task, you are presented with an overview of the requested changes before submitting the task for execution. When it’s submitted, the task first enters a Requested status while the configuration is evaluated, before either being moved to In Progress if the task is valid or Failed if it’s invalid or blocked by running instances.
When the capacity task has successfully completed and the capacity pools for the asset are updated, you can validate this by returning to the rack view within the Outpost console, or by using the CLI/API. The following is an example using the list-assets CLI command:
Only a single capacity task for an asset can be executing at any given time. If you attempt to create a second capacity task for the same asset while the original is still in a Requesting or In Progress status, then the submission of the task fails. However, you can submit multiple capacity tasks for unique assets within the same Outpost. For example, using the CLI commands, you could execute a single script to change the capacity configuration of all assets within an Outpost through individual asset level capacity tasks.
Considerations
- Make sure that if you’re specifying instance type in your launch templates, then this instance type is available on multiple assets if your workload needs to be resilient against host failures.
- Understand which failure scenarios could exist within your environment, and plan for how each one should be handled. Failure planning is essential for maintaining workload uptime in production environments.
- Capacity tasks can only be executed from the AWS account that owns the Outpost. If Outpost resources are shared to workload accounts through AWS Resource Access Manager (AWS RAM), then these accounts can’t submit capacity tasks.
- You can manipulate your capacity configuration to control instance placement at launch. If only certain assets support the instance size and type you want to deploy, then your instance must be launched on one of those assets.
- If executing capacity tasks through CLI commands, make sure that your CLI has been updated to the latest version. We have updated our CLI with this feature release to include commands for capacity tasks, and they fail if running on outdated versions.
Conclusion
This post demonstrates how to use Asset Level Capacity Management with your AWS Outposts, and reviews considerations for maintaining a highly available capacity configuration. For more information on how to manage and monitor your capacity configuration on Outposts, see the Capacity management for AWS Outposts user guide and the Capacity planning section of the Outposts High Availability Design and Architecture Considerations whitepaper. Reach out to your AWS account team, or fill out this form to learn more about Outposts and self-service capacity management.