# Cluster size in AWS PCS
<a name="working-with_clusters_size"></a>

AWS PCS provides highly available and secure clusters, while automating key tasks such as patching, node provisioning, and updates.

When you create a cluster, you select a size for it based on two factors:
+  The number of compute nodes it will manage 
+  The number of jobs tracked by the controller at any given time 

**Note**  
The job count includes running, pending, and recently completed jobs. Completed jobs remain tracked by the controller for a short period before being purged. During high job throughput, this can cause the total tracked job count to exceed the number of active jobs you observe.

**Important**  
You can't change the cluster size after you create the cluster. If you need to change the size, you must create a new cluster.


| Slurm cluster size | Number of instances managed | Number of jobs tracked by the controller | 
| --- | --- | --- | 
| Small | Up to 32 | Up to 256 | 
| Medium | Up to 512 | Up to 8192 | 
| Large | Up to 2048 | Up to 16384 | 

**Examples**
+ If your cluster will have up to 24 managed instances and run up to 100 jobs, choose **Small**.
+ If your cluster will have up to 24 managed instances and run up to 1000 jobs, choose **Medium**.
+ If your cluster will have up to 1000 managed instances and run up to 100 jobs, choose **Large**.
+ If your cluster will have up to 1000 managed instances and run up to 10,000 jobs, choose **Large**.