Atlas Sizing and Tier Selection

Choosing the correct Atlas cluster tier and configuration is an important step in setting up a successful production MongoDB deployment. You can always modify a cluster at a later time, but getting started with the right configuration is possible with a few calculations based on your data set size and network requirements.

You can also configure your cluster to automatically scale its cluster tier, storage capacity, or both in response to cluster usage, thereby reducing the manual maintenance required for your cluster. To learn more, see Cluster Auto-Scaling.

Recommend M30 or Larger Clusters for Production Use

M30 and higher clusters are recommended for production environments. You can use M10 and M20 clusters as production environments for low-traffic applications, but these tiers are recommended for development environments.

Cluster tiers M10 and greater support Cluster Auto-Scaling. When auto-scaling is enabled, Atlas automatically scales your cluster tier, storage capacity, or both in response to cluster usage. Consider enabling auto-scaling to allow your cluster to adapt to your current workload and reduce the need to make manual optimizations.

  • Cluster storage scaling automatically increases your cluster storage capacity when 90% of disk capacity is used. This setting is on by default to help ensure that your cluster can always support sudden influxes of data.
  • Cluster tier scaling automatically scales your cluster tier up or down in response to various cluster metrics. You must manually opt-in to cluster tier scaling.

    When you enable cluster tier scaling, you set:

    • A maximum cluster tier to which your cluster can automatically scale up, and
    • (Optional) A minimum tier to which your cluster can scale down.

Memory refers to the total physical RAM available on each data bearing node of your Atlas cluster. For example, an M30 standard replica set is configured with 8 GB RAM on each of the 3 data bearing nodes.

Atlas uses the WiredTiger storage engine.

  • By default, for M40 or larger clusters, WiredTiger dedicates 50% of physical RAM for the WiredTiger cache. The other 50% is reserved for in-memory operations such as sorts and calculations, the underlying operating system and other system services.
  • By default, for M30 or smaller clusters, WiredTiger dedicates 25% of physical RAM for the WiredTiger cache.

To learn more about memory use, see WiredTiger and Memory Use.

MongoDB uses the WiredTiger cache to hold most recently used data and indexes. The working set is the sum of all the indexes plus the set of documents that are accessed frequently. If your working set fits in RAM, then MongoDB can serve queries from memory, which provides the fastest query response times.

To estimate the size of the working set, you can either perform a calculation using the information obtained from db.stats for the total index space and assume a percentage of your data space will be accessed frequently, or you can estimate your memory requirements based on educated assumptions.

Using the Atlas sample data sets, we will calculate the memory requirements to run all these databases in a single Atlas cluster. The following JavaScript program returns database information for your cluster:

var totalIndexSize = 0;
var totalDataSize = 0;
var reservedDBs = ["admin","config","local"];
// Switch to admin database and get list of databases.
db = db.getSiblingDB("admin");
dbs = db.runCommand({ "listDatabases": 1 }).databases;
// Iterate through each database and get its stats.
dbs.forEach(function(database) {
if (reservedDBs.includes(
db = db.getSiblingDB(;
print("Obtaining stats for " +;
var stats = db.stats();
totalIndexSize += (stats.indexSize / (1024*1024*1024)) ;
totalDataSize += (stats.dataSize / (1024*1024*1024)) ;
print ("Total data size in GB: " + totalDataSize.toFixed(2));
print ("Total index size in GB: " + totalIndexSize.toFixed(2));

This program returns the following results:

Obtaining stats for sample_airbnb
Obtaining stats for sample_geospatial
Obtaining stats for sample_mflix
Obtaining stats for sample_supplies
Obtaining stats for sample_training
Obtaining stats for sample_weatherdata
Total data size in GB: 0.32
Total index size in GB: 0.02

To run these databases completely in memory, you would need a minimum of 0.68 GB of physical RAM, as WiredTiger uses 50% of the physical RAM and we need at least 0.34 GB to fit the working set in memory.

A realistic production cluster would have a much higher data and index size and it may not be practical, or not a business or performance requirement, to run the complete data set and indexes in memory. Let’s look at another scenario.

A popular mobile game has 512 GB of data and 32 GB of indexes. The game’s internal system data occupies 16 GB, and the rest is player profile data. A player's profile is required to be in memory while the player is active in the game. About 25% of all players are active at any point in time. The system data is used constantly and it must fit completely in RAM for optimal game performance. All indexes must also fit in RAM for the fastest query response time. The memory sizing is as follows:

RAM Requirements
RAM for WiredTiger Cache
System: 16 GB
100% in RAM
16 GB
Index: 32 GB
100% in RAM
32 GB
Player Profiles: 496 GB
25% in RAM
124 GB

Given these requirements, you can expect an average working set to require 172 GB of RAM.

WiredTiger dedicates 50% of physical RAM to the WiredTiger cache, so the minimum total physical RAM required to accommodate your working set is twice the working set.

In this example, you need at least 344 GB of physical RAM to accommodate the WiredTiger cache and a 172 GB working set. The following table lists suitable Atlas cluster tiers:

Service Provider
Possible Cluster Tiers
  • M300 384 GB RAM
  • M400 488 GB RAM
  • M700 768 GB RAM
M300 has sufficient RAM, and there is room to scale vertically to higher cluster tiers.
  • M300 360 GB RAM
M300 has sufficient RAM, but there are no higher tiers available if vertical scaling is required.
  • M300 384 GB RAM
  • M400 432 GB RAM
M300 has sufficient RAM, and there is room to scale vertically to a higher cluster tier.

If you select a cluster tier without sufficient RAM, such as an Azure M200 with 256 GB RAM, sharding is required.

All network traffic between cluster nodes and between clients consuming data from your Atlas cluster impact network bandwidth. For purposes of cluster sizing, consider the maximum traffic that any node on your cluster will bear and use this as the basis for selecting an adequate Atlas cluster tier.

Downstream data transfer rates from your cluster to client applications can be calculated as the sum of the total documents returned over a period of time. If you are reading only from the primary node, this is the only calculation you need to do. If your applications read from secondary nodes as well, you can divide this number by the number of nodes that can serve read operations.

You can find the average document size for a database with the db.stats method. Multiply the average document size (avgObjSize) by the number of documents served per second to estimate your bandwidth requirements.


10 KB average document size

100,000 documents per second served

10 KB * 100,000 = 1 GB per second

Atlas instances provide faster bandwidth speeds at the higher tiers. Instances which are backed by AWS provide up to 10 gigabits per second at the M30 level, while the M200 level provides up to 25 gigabits per second.

When M10 and M20 clusters hosted on AWS experience periods of high usage, they can temporarily increase performance through bursting. During periods of low usage, M10 and M20 clusters accumulate EC2 instance credits, and these credits are depleted when bursting occurs. The System: CPU (Steal) % is alert is triggered when the EC2 instance credit balance is exhausted.

An Atlas cluster can support a number of client connections which is determined by its cluster tier. M30 clusters support up to 2000 simultaneous connections, while M200 clusters support up to 128,000 simultaneous connections.

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