Redis maxmemory Explained: How to Limit Memory Usage and Prevent OOM Errors

Introduction

The maxmemory directive controls the maximum amount of memory that Redis can use. It is one of the most important configuration options because Redis primarily stores data in memory.

Without a memory limit, Redis may continue allocating memory until the operating system runs out of available RAM, potentially causing performance problems or triggering the Linux Out Of Memory (OOM) killer.

This guide explains how maxmemory works, how to configure it, and best practices for production environments.

Test Environment

Operating System : AlmaLinux 10
PHP Version      : PHP 8.3
Web Server       : Nginx
PHP-FPM          : Enabled
Architecture     : x86_64
Last Tested      : July 2026

Default Value

Many Redis installations have no memory limit configured by default.

maxmemory 0

A value of 0 means Redis does not enforce a maximum memory limit.

Syntax

maxmemory

Examples:

maxmemory 256mb

maxmemory 1gb

maxmemory 4gb

Redis supports several memory units:

| Unit | Example |
| ---- | ------- |
| KB   | 512kb   |
| MB   | 512mb   |
| GB   | 4gb     |

Why Is maxmemory Important?

Redis stores active data in RAM.

If memory usage continues to grow without limits, several problems may occur:

The operating system may run out of memory.

Other applications may become unstable.

Linux may terminate Redis using the OOM killer.

Server performance may degrade significantly.

Setting an appropriate memory limit helps maintain predictable performance.

How Redis Behaves

Suppose a server has:

RAM: 8 GB

Redis configuration:

maxmemory 6gb

Redis can use approximately 6 GB for storing data.

Once memory reaches the configured limit, Redis follows the configured eviction policy (maxmemory-policy) to decide how to handle new writes.

How to Configure maxmemory

Open the Redis configuration file.

sudo nano /etc/redis.conf

Locate or add:

maxmemory 2gb

Save the file.

Restart Redis.

sudo systemctl restart redis

Verify the Configuration

Run:

redis-cli CONFIG GET maxmemory

Example output:

1) "maxmemory"

2) "2147483648"

The value is displayed in bytes.

Change the Setting Without Restarting

Redis allows runtime configuration.

redis-cli CONFIG SET maxmemory 1gb

Verify:

redis-cli CONFIG GET maxmemory

Note that runtime changes may not persist after restarting unless the configuration file is also updated.

Choosing the Right Value

There is no universal setting.

Consider:

Total server memory.

Operating system requirements.

Other running applications.

Expected dataset size.

Future growth.

Example recommendations:

| Server RAM | Suggested maxmemory |
| ---------- | ------------------- |
| 2 GB       | 1 GB                |
| 4 GB       | 2–3 GB              |
| 8 GB       | 5–6 GB              |
| 16 GB      | 10–12 GB            |
| 32 GB      | 22–26 GB            |

Always leave sufficient memory for the operating system and other services.

Common Mistakes

Setting maxmemory Too High

Redis may leave insufficient memory for the operating system, increasing the risk of swapping or OOM termination.

Forgetting the Eviction Policy

maxmemory alone does not determine what happens when memory is exhausted.

The maxmemory-policy directive controls whether Redis evicts keys or rejects write operations.

Ignoring Memory Fragmentation

Actual memory consumption may exceed the configured dataset size because of allocator overhead and fragmentation.

Monitor memory usage regularly.

Best Practices

Always configure maxmemory for production servers.

Leave free RAM for the operating system.

Combine maxmemory with an appropriate maxmemory-policy.

Monitor memory usage using INFO memory.

Test memory limits before deploying to production.

Avoid using swap for Redis whenever possible.

Related Configuration

Redis maxmemory-policy Explained

Redis INFO memory

Redis appendonly Explained

Redis save Explained

Redis persistence Explained

Conclusion

The maxmemory directive defines the maximum amount of memory Redis can use and is essential for maintaining server stability. A carefully chosen memory limit helps prevent out-of-memory conditions while ensuring Redis delivers consistent performance.

For most production deployments, maxmemory should always be configured together with a suitable eviction policy and regular memory monitoring to achieve the best balance between performance and reliability.