Managing reserved memory for Valkey and Redis OSS

Reserved memory is memory set aside for nondata use. When performing a backup or failover, Valkey and Redis OSS use available memory to record write operations to your cluster while the cluster's data is being written to the .rdb file. If you don't have sufficient memory available for all the writes, the process fails. Following, you can find information on options for managing reserved memory for ElastiCache (Redis OSS) and how to apply those options.

How Much Reserved Memory Do You Need?

If you are running a version of Redis OSS before 2.8.22, reserve more memory for backups and failovers than if you are running Redis OSS 2.8.22 or later. This requirement is due to the different ways that ElastiCache (Redis OSS) implements the backup process. The rule of thumb is to reserve half of a node type's maxmemory value for Redis OSS overhead for versions before 2.8.22, and one-fourth for Redis OSS versions 2.8.22 and later.

Due to different ways that ElastiCache implements the backup and replication process, the rule of thumb is to reserve 25% of a node type's maxmemory value by using the reserved-memory-percent parameter. This is the default value and recommended for most cases.

When burstable micro and small instance types are operating near the maxmemory limits, they may experience swap usage. To improve the operational reliability on these instance types during backup, replication and high traffic, we recommend increasing the value of the reserved-memory-percent parameter up to 30% on small instance types, and up to 50% on micro instance types.

For write-heavy workloads on ElastiCache clusters with data tiering, we recommend increasing the reserved-memory-percent to up to 50% of the node's available memory.

For more information, see the following:

• Ensuring you have enough memory to make a Valkey or Redis OSS snapshot

• How synchronization and backup are implemented

• Data tiering in ElastiCache

Parameters to Manage Reserved Memory

As of March 16, 2017, Amazon ElastiCache provides two mutually exclusive parameters for managing your Valkey or Redis OSS memory, reserved-memory and reserved-memory-percent. Neither of these parameters is part of the Valkey or Redis OSS distribution.

Depending upon when you became an ElastiCache customer, one or the other of these parameters is the default memory management parameter. This parameter applies when you create a new Valkey or Redis OSS cluster or replication group and use a default parameter group.

• For customers who started before March 16, 2017 – When you create a Redis OSS cluster or replication group using the default parameter group, your memory management parameter is reserved-memory. In this case, zero (0) bytes of memory are reserved.

• For customers who started on or after March 16, 2017 – When you create a Valkey or Redis OSS cluster or replication group using the default parameter group, your memory management parameter is reserved-memory-percent. In this case, 25 percent of your node's maxmemory value is reserved for nondata purposes.

After reading about the two Valkey or Redis OSS memory management parameters, you might prefer to use the one that isn't your default or with nondefault values. If so, you can change to the other reserved memory management parameter.

To change the value of that parameter, you can create a custom parameter group and modify it to use your preferred memory management parameter and value. You can then use the custom parameter group whenever you create a new Valkey or Redis OSS cluster or replication group. For existing clusters or replication groups, you can modify them to use your custom parameter group.

For more information, see the following:

• Specifying Your Reserved Memory Management Parameter

• Creating an ElastiCache parameter group

• Modifying an ElastiCache parameter group

• Modifying an ElastiCache cluster

• Modifying a replication group

The reserved-memory Parameter

Before March 16, 2017, all ElastiCache (Redis OSS) reserved memory management was done using the parameter reserved-memory. The default value of reserved-memory is 0. This default reserves no memory for Valkey or Redis OSS overhead and allows Valkey or Redis OSS to consume all of a node's memory with data.

Changing reserved-memory so you have sufficient memory available for backups and failovers requires you to create a custom parameter group. In this custom parameter group, you set reserved-memory to a value appropriate for the Valkey or Redis OSS version running on your cluster and cluster's node type. For more information, see How Much Reserved Memory Do You Need?

The parameter reserved-memory is specific to ElastiCache and isn't part of the general Redis OSS distribution.

The following procedure shows how to use reserved-memory to manage the memory on your Valkey or Redis OSS cluster.

To reserve memory using reserved-memory

1. Create a custom parameter group specifying the parameter group family matching the engine version you’re running—for example, specifying the redis2.8 parameter group family. For more information, see Creating an ElastiCache parameter group.

   aws elasticache create-cache-parameter-group \
      --cache-parameter-group-name redis6x-m3xl \
      --description "Redis OSS 2.8.x for m3.xlarge node type" \
      --cache-parameter-group-family redis6.x

2. Calculate how many bytes of memory to reserve for Valkey or Redis OSS overhead. You can find the value of maxmemory for your node type at Redis OSS node-type specific parameters.

3. Modify the custom parameter group so that the parameter reserved-memory is the number of bytes you calculated in the previous step. The following AWS CLI example assumes you’re running a version of Redis OSS before 2.8.22 and need to reserve half of the node’s maxmemory. For more information, see Modifying an ElastiCache parameter group.

   aws elasticache modify-cache-parameter-group \
      --cache-parameter-group-name redis28-m3xl \
      --parameter-name-values "ParameterName=reserved-memory, ParameterValue=7130316800"

   You need a separate custom parameter group for each node type that you use, because each node type has a different maxmemory value. Thus, each node type needs a different value for reserved-memory.

4. Modify your Redis OSS cluster or replication group to use your custom parameter group.

   The following CLI example modifies the cluster my-redis-cluster to use the custom parameter group redis28-m3xl beginning immediately. For more information, see Modifying an ElastiCache cluster.

   aws elasticache modify-cache-cluster \
      --cache-cluster-id my-redis-cluster \
      --cache-parameter-group-name redis28-m3xl \
      --apply-immediately

   The following CLI example modifies the replication group my-redis-repl-grp to use the custom parameter group redis28-m3xl beginning immediately. For more information, Modifying a replication group.

   aws elasticache modify-replication-group \
      --replication-group-id my-redis-repl-grp \
      --cache-parameter-group-name redis28-m3xl \
      --apply-immediately

The reserved-memory-percent parameter

On March 16, 2017, Amazon ElastiCache introduced the parameter reserved-memory-percent and made it available on all versions of ElastiCache (Redis OSS). The purpose of reserved-memory-percent is to simplify reserved memory management across all your clusters. It does so by enabling you to have a single parameter group for each parameter group family (such as redis2.8) to manage your clusters' reserved memory, regardless of node type. The default value for reserved-memory-percent is 25 (25 percent).

The parameter reserved-memory-percent is specific to ElastiCache and isn't part of the general Redis OSS distribution.

If your cluster is using a node type from the r6gd family and your memory usage reaches 75 percent, data-tiering will automatically be triggered. For more information, see Data tiering in ElastiCache.

To reserve memory using reserved-memory-percent

To use reserved-memory-percent to manage the memory on your ElastiCache (Redis OSS) cluster, do one of the following:

• If you are running Redis OSS 2.8.22 or later, assign the default parameter group to your cluster. The default 25 percent should be adequate. If not, take the steps described following to change the value.

• If you are running a version of Redis OSS before 2.8.22, you probably need to reserve more memory than reserved-memory-percent's default 25 percent. To do so, use the following procedure.

To change the percent value of reserved-memory-percent

1. Create a custom parameter group specifying the parameter group family matching the engine version you’re running—for example, specifying the redis2.8 parameter group family. A custom parameter group is necessary because you can't modify a default parameter group. For more information, see Creating an ElastiCache parameter group.

   aws elasticache create-cache-parameter-group \
      --cache-parameter-group-name redis28-50 \
      --description "Redis OSS 2.8.x 50% reserved" \
      --cache-parameter-group-family redis2.8

   Because reserved-memory-percent reserves memory as a percent of a node’s maxmemory, you don't need a custom parameter group for each node type.

2. Modify the custom parameter group so that reserved-memory-percent is 50 (50 percent). For more information, see Modifying an ElastiCache parameter group.

   aws elasticache modify-cache-parameter-group \
      --cache-parameter-group-name redis28-50 \
      --parameter-name-values "ParameterName=reserved-memory-percent, ParameterValue=50"

3. Use this custom parameter group for any Redis OSS clusters or replication groups running a version of Redis OSS older than 2.8.22.

   The following CLI example modifies the Redis OSS cluster my-redis-cluster to use the custom parameter group redis28-50 beginning immediately. For more information, see Modifying an ElastiCache cluster.

   aws elasticache modify-cache-cluster \
      --cache-cluster-id my-redis-cluster \
      --cache-parameter-group-name redis28-50 \
      --apply-immediately

   The following CLI example modifies the Redis OSS replication group my-redis-repl-grp to use the custom parameter group redis28-50 beginning immediately. For more information, see Modifying a replication group.

   aws elasticache modify-replication-group \
      --replication-group-id my-redis-repl-grp \
      --cache-parameter-group-name redis28-50 \
      --apply-immediately

Specifying Your Reserved Memory Management Parameter

If you were a current ElastiCache customer on March 16, 2017, your default reserved memory management parameter is reserved-memory with zero (0) bytes of reserved memory. If you became an ElastiCache customer after March 16, 2017, your default reserved memory management parameter is reserved-memory-percent with 25 percent of the node's memory reserved. This is true no matter when you created your ElastiCache (Redis OSS) cluster or replication group. However, you can change your reserved memory management parameter using either the AWS CLI or ElastiCache API.

The parameters reserved-memory and reserved-memory-percent are mutually exclusive. A parameter group always has one but never both. You can change which parameter a parameter group uses for reserved memory management by modifying the parameter group. The parameter group must be a custom parameter group, because you can't modify default parameter groups. For more information, see Creating an ElastiCache parameter group.

To specify reserved-memory-percent

To use reserved-memory-percent as your reserved memory management parameter, modify a custom parameter group using the modify-cache-parameter-group command. Use the parameter-name-values parameter to specify reserved-memory-percent and a value for it.

The following CLI example modifies the custom parameter group redis32-cluster-on so that it uses reserved-memory-percent to manage reserved memory. A value must be assigned to ParameterValue for the parameter group to use the ParameterName parameter for reserved memory management. For more information, see Modifying an ElastiCache parameter group.

aws elasticache modify-cache-parameter-group \
   --cache-parameter-group-name redis32-cluster-on \
   --parameter-name-values "ParameterName=reserved-memory-percent, ParameterValue=25"

To specify reserved-memory

To use reserved-memory as your reserved memory management parameter, modify a custom parameter group using the modify-cache-parameter-group command. Use the parameter-name-values parameter to specify reserved-memory and a value for it.

The following CLI example modifies the custom parameter group redis32-m3xl so that it uses reserved-memory to manage reserved memory. A value must be assigned to ParameterValue for the parameter group to use the ParameterName parameter for reserved memory management. Because the engine version is newer than 2.8.22, we set the value to 3565158400 which is 25 percent of a cache.m3.xlarge’s maxmemory. For more information, see Modifying an ElastiCache parameter group.

aws elasticache modify-cache-parameter-group \
   --cache-parameter-group-name redis32-m3xl \
   --parameter-name-values "ParameterName=reserved-memory, ParameterValue=3565158400"