# Capacity Reservations in AWS PCS
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 You can reserve Amazon EC2 capacity in a specific Availability Zone and for a specific duration using On-Demand Capacity Reservations or Amazon EC2 Capacity Blocks for ML to make sure that you have the necessary compute capacity available when you need it. 

 **On-Demand Capacity Reservations (ODCRs)** let you reserve compute capacity for your Amazon EC2 instances in a specific Availability Zone for any duration. You can create and cancel reservations at any time, with no long-term commitments or upfront payments. ODCRs are ideal when you need flexible capacity reservations that you can modify as your requirements change. For more information, see [On-Demand Capacity Reservations](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-capacity-reservations.html) in the *Amazon Elastic Compute Cloud User Guide*. 

 **Amazon EC2 Capacity Blocks for ML** let you reserve GPU-based accelerated computing instances for future use, up to 8 weeks in advance. You can reserve blocks of 1-64 instances for durations from 1 day to 6 months. Capacity Blocks are ideal for machine learning workloads that require guaranteed access to GPU capacity at specific times. 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*. 

**Topics**
+ [Using ODCRs with AWS PCS](capacity-reservations-odcr.md)
+ [Using I-ODCRs with AWS PCS](capacity-reservations-iodcr.md)
+ [Using Amazon EC2 Capacity Blocks for ML with AWS PCS](capacity-blocks.md)