# Using Amazon EC2 Capacity Blocks for ML with AWS PCS
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Amazon EC2 Capacity Blocks for ML is an Amazon EC2 purchasing option that enables you to pay in advance to reserve GPU-based accelerated computing instances within a specific date and time range to support short duration workloads. Instances that run inside a Capacity Block are automatically placed close together inside Amazon EC2 UltraClusters, for low-latency, petabit-scale, non-blocking networking. For more information, see [Capacity Blocks for ML](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-blocks.html) in the *Amazon Elastic Compute Cloud User Guide*.

You can use a launch template to have AWS PCS use a Capacity Block when it launches instances for a compute node group.

**Note**  
AWS PCS introduced support for Capacity Blocks since Slurm version 24.05.

## Limitations
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+ AWS PCS only supports Capacity Blocks with P6-B300, P6-B200, P5en, P5e, P5, and P4d instance families.
+ You can only associate a compute node group with 1 Capacity Block at a time.
+ You can't associate a compute node group with a capacity reservation group that combines multiple Capacity Blocks.
+ Capacity Blocks must be in a `scheduled` or `active` state to use with AWS PCS. You can't use Capacity Blocks in other states, such as `payment-failed`. For more information, see [View Capacity Blocks](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/capacity-blocks-view.html) in the *Amazon Elastic Compute Cloud User Guide*.
+ For P6 and P5 instance types please refer to the relevant AWS documentation: [Software Requirements for P6 Instances](https://docs.aws.amazon.com/dlami/latest/devguide/p6-support-dlami.html#dlami-support-p6), [Maximize network bandwidth on Amazon EC2 instances with multiple network cards](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/efa-acc-inst-types.html)

## Capacity Block expiration
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Capacity Blocks are limited to a specific date and time range. When a Capacity Block expires:
+ The compute node group associated with that Capacity Block continues to exist and remains associated with the same queues.
+ All instances in the compute node group are terminated and active jobs might fail, based on your Slurm settings.
+ AWS PCS can't launch new instances in the compute node group.
+ All queued or newly submitted jobs remain in pending state until another compute node group is attached to the queue or you update the compute node group to use a new launch template that specifies a new Capacity Block.