Cluster Auto-Scaling

Feature Availability

This feature is available for M10+ clusters using the General storage class.

You can configure your Atlas cluster to automatically scale its cluster tier, storage capacity, or both in response to cluster usage. Cluster auto-scaling removes the need to write scripts or use consulting services to analyze cluster usage to make scaling decisions.

Auto-scaling works on a rolling basis, meaning the process does not incur any downtime.

How Auto-Scaling Works

The following sections describe how Atlas automatically scales your cluster tier and storage capacity, and the metrics considered for each scaling procedure.

Scaling Cluster Tier

Atlas analyzes the following cluster metrics to determine when to scale a cluster, and whether to scale the cluster tier up or down:

  • CPU Utilization
  • Memory Utilization

To learn more about how to monitor cluster metrics, see Monitor a Cluster.

Before scaling your cluster tier, Atlas checks that the cluster would not be in a tier outside of your specified Minimum and Maximum Cluster Size range.

Scaling Up a Cluster Tier

If the next highest cluster tier is within your Maximum Cluster Size range, Atlas scales the cluster up to the next tier if one of the following is true for the current cluster:

  • Average CPU Utilization has exceeded 75% for the past hour, or
  • Memory Utilization has exceeded 75% for the past hour.

Scaling Down a Cluster Tier

If the next lowest cluster tier is within your Minimum Cluster Size range, Atlas scales the cluster down to the next lowest tier if both of the following are true:

  • Based on data collected from the past 72 hours, the desired CPU Utilization and Memory Utilization on the target cluster tier will not exceed 50%, and
  • The cluster has not been scaled down in the past 72 hours.

Scaling Cluster Storage

When cluster storage auto-scaling is enabled, Atlas automatically increases cluster storage when disk usage reaches 90%. The scaling behavior differs by cloud provider:

  • On AWS and GCP clusters, Atlas increases cluster storage capacity to achieve 70% disk utilization. For information on AWS storage change limitations and how Atlas works around them, see Changing Storage Capacity on AWS.
  • On Azure clusters, Atlas doubles the current amount of cluster storage.


When you increase the storage capacity of a cluster, Atlas increases the cluster’s oplog size. Atlas scales the oplog to 5% of the cluster capacity, not to exceed 50 GB. NVMe storage requires an oplog which is 10% of the storage capacity. Atlas doesn’t change the oplog size if it exceeds 5% of the new storage capacity (10% in the case of NVMe storage).

As cluster storage capacity decreases, Atlas doesn’t change the oplog size unless it exceeds a certain maximum determined according to MongoDB best practices.

Configure Auto-Scaling Options

You can configure auto-scaling options when you create or modify a cluster.

Atlas displays auto-scaling options in the Auto-scale section of the cluster builder:

Image showing the auto-scaling cluster options.

Auto-Scale Cluster Tier

In the Cluster tier section of the Auto-scale options, you can specify the minimum and maximum sizes to which your cluster can automatically scale.

Auto-scaling is disabled by default. To enable auto-scaling:

  1. In the Auto-Scale section, check Cluster tier.
  2. Select a Maximum Cluster Size from the dropdown.

When you enable auto-scaling, by default your cluster can only automatically scale up to a higher cluster tier. To enable auto-scaling to a lower cluster tier:

  1. Check Allow cluster to be scaled down.
  2. Select a Minimum Cluster Size from the dropdown.


  • Automatically scaling down to a lower cluster tier may leave your cluster unable to handle your workload. Only allow the cluster to be scaled down if you have a clear understanding of your cluster’s workload. Ensure that you account for any possible spikes or dips in cluster activity.
  • You cannot scale to a cluster smaller than M10.

Auto-Scale Cluster Storage

Cluster storage scaling is enabled by default. To opt out of cluster storage scaling, un-check the Storage checkbox in the Auto-scale section.

For more information on cluster storage, see Customize Cluster Storage.

Acknowledge Auto-Scaling Events

When an auto-scaling event occurs:

  • Atlas logs the event in the project Activity Feed. For more information on the Activity Feed, see View All Activity.
  • Atlas sends an email to all cluster members with information on the event. The email includes the original cluster tier and the new cluster tier after auto-scaling has occurred.