# Managing reserved memory for Valkey and Redis OSS
<a name="redis-memory-management"></a>

Each host type has an amount of memory dedicated for Valkey and Redis OSS. This is the advertised memory. The `reserved-memory-percent` parameter sets aside part of this memory for nondata uses:
+ Client output buffers on replicas
+ Memory lost to fragmentation
+ Copy-on-write memory used by forked bgsave

The `maxmemory` value equals the advertised memory minus the reserve. For example, with 13.5 GB of advertised memory and a 25% reserve, the `maxmemory` is about 10.1 GB.

**Important**  
The `reserved-memory-percent` defaults to 25%. Do not reduce this value. A lower reserve can cause data loss, especially during upgrades or other changes to your cluster.

**Topics**
+ [How much memory to reserve](#redis-memory-management-how-much)
+ [Configuring reserved memory](#redis-memory-management-configure)

## How much memory to reserve
<a name="redis-memory-management-how-much"></a>

The `reserved-memory-percent` parameter defaults to 25%. Do not reduce this value. However, there are situations where you can benefit from increasing the memory reserve.

Micro and small instances  
The micro and small instances are sized for light activity without the burden of replication. To improve operational reliability of these instances for production workloads, increase the memory reserve to 30% on small instances and 50% on micro instances.

Forcing a forked bgsave or full sync  
During a traditional forked bgsave or full sync, Valkey and Redis OSS memory usage can double (or more), depending on the traffic load and pattern. If `FreeableMemory` is insufficient to run a traditional forked operation, Amazon ElastiCache automatically switches to a forkless save mechanism. This forkless mechanism uses much less memory, but can add client latencies — especially with very large key/value items. If these latencies are impactful for your use case, increase the memory reserve to ensure enough memory for a traditional forked save.

If instructed by AWS Support  
Unusual traffic patterns or requirements can result in unique memory needs. Increase `reserved-memory-percent` if directed by AWS Support.

## Configuring reserved memory
<a name="redis-memory-management-configure"></a>

The `reserved-memory-percent` parameter is specific to ElastiCache. It isn't part of the Valkey or Redis OSS open source code. To change this value, use a custom parameter group. You can't modify default parameter groups.

If 25% works for you, no changes are needed. To use a different value, follow these steps.

**To change the reserved-memory-percent value**

1. Run the following AWS CLI command to create a custom parameter group. Set the family to match your engine version. For more information about creating parameter groups, see [Creating an ElastiCache parameter group](ParameterGroups.Creating.md).

   ```
   aws elasticache create-cache-parameter-group \
      --cache-parameter-group-name {{my-parameter-group}} \
      --description "{{Custom reserved memory settings}}" \
      --cache-parameter-group-family {{redis7}}
   ```

   You need only one custom parameter group per engine family.

1. Set `reserved-memory-percent` to the value you want. For more information about modifying parameter groups, see [Modifying an ElastiCache parameter group](ParameterGroups.Modifying.md).

   ```
   aws elasticache modify-cache-parameter-group \
      --cache-parameter-group-name {{my-parameter-group}} \
      --parameter-name-values "ParameterName=reserved-memory-percent, ParameterValue={{30}}"
   ```

1. Apply the custom parameter group to your cluster or replication group.

   To modify a cluster, run the following AWS CLI command. For more information about modifying clusters, see [Modifying an ElastiCache cluster](Clusters.Modify.md).

   ```
   aws elasticache modify-cache-cluster \
      --cache-cluster-id {{my-cluster}} \
      --cache-parameter-group-name {{my-parameter-group}} \
      --apply-immediately
   ```

   To modify a replication group, run the following AWS CLI command. For more information about modifying replication groups, see [Modifying a replication group](Replication.Modify.md).

   ```
   aws elasticache modify-replication-group \
      --replication-group-id {{my-replication-group}} \
      --cache-parameter-group-name {{my-parameter-group}} \
      --apply-immediately
   ```