About scaling instance capacity
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This page describes how your Memorystore for Valkey instance behaves during scaling.

You can scale the capacity for your instance in the following ways:

You can change the number of shards for your instance. This is horizontal scaling. You can scale an instance horizontally in one of the following ways:
- Scale out: increase the capacity of your instance, which gives the instance more memory and processing power to handle a larger volume of data or traffic. Your instance's capacity is determined by the number of shards in your instance.
  
  By scaling your instance out, your application can handle an increased demand without performance degradation. To scale an instance out, add shards to the instance.
- Scale in: reduce the capacity of your instance, which decreases both the processing power and the amount of memory available to store data. This occurs when your application's data demands decrease and you need to lower your resource usage to save costs. To scale an instance in, reduce the number of shards in the instance.
Note: If you specify Cluster Mode Disabled when you create an instance, then Memorystore for Valkey doesn't partition the instance, but hosts it on a single shard. Therefore, horizontal scaling doesn't apply for the instance.
You can change the node type for your instance. This is vertical scaling. You can scale an instance vertically in one of the following ways:
- Scale up: increase the capacity of your instance. Your instance's capacity is determined by your instance's node type. To scale an instance up, change the node type to a larger node type. For example, scale your instance up from a standard-small node type to a highmem-medium node type.
- Scale down: reduce the capacity of your instance. To scale an instance down, change the node type to a smaller node type. For example, scale your instance down from a highmem-medium node type to a standard-small node type.
  
  Important: If you want to scale the node type for your instance down, and you haven't overwritten the default settings for the original node type, then Memorystore for Valkey modifies the default settings to those of the scaled node type.

Impact of scaling

During a scaling operation, the availability of your instance isn't impacted. However, when you change the number of shards in your instance, Memorystore for Valkey rebalances the instance's keyspace. This might cause increased latency during the scaling operation.

Also, the impact of scaling your instance vertically by changing the node type is similar to what happens with a maintenance operation.

Failure scenarios

If you encounter an error during the scaling operation, then it's likely because of one of the following scenarios:

You want to update the shard count for your instance, but you don't have enough free memory for the shared-core-nano node type for your instance.

To resolve this issue, you can either scale your instance up or free memory from the nodes of your instance. To free memory, do the following:
1. Connect to a Memorystore for Valkey instance.
2. To get details about the instance, including the IP address and port number, use the gcloud memorystore instances describe command.
3. Make a note of the IP address and port number of the instance.
4. To get information about the nodes of the instance, use the following command:
```
valkey-cli -h IP_ADDRESS -p PORT_NUMBER cluster nodes
```
  Replace IP_ADDRESS and PORT_NUMBER with the values that you noted in the previous step.
5. Make a note of the IP address and port number of a node that doesn't have enough free space. This node's ID matches an ID that appears in the error.
6. To connect to this node, use the following command:
```
valkey-cli -h IP_ADDRESS -p PORT_NUMBER
```
  Replace IP_ADDRESS and PORT_NUMBER with the values that you noted in the previous step.
7. At the prompt, enter the info memory command.
  
  In the output, values for the used_memory and maxmemory parameters appear. used_memory is how much memory the node uses and maxmemory is how much memory is available for the node.
8. Divide the value of the used_memory parameter by the value of the maxmemory parameter, and verify that the quotient is greater than 98%.
9. To free up space for the node, delete some of the node's keys.
10. Enter the info memory command again. In the output, the value of the used_memory parameter is smaller.
11. Divide the value of the used_memory parameter by the value of the maxmemory parameter, and verify that the quotient is now less than 98%. If it isn't, then delete more keys.
12. For any other nodes that don't have enough free space, repeat steps 4 - 11 of this procedure.
You scaled to a smaller shard count that doesn't have the capacity to hold all of the keys that Memorystore for Valkey stores in the original instance. To resolve this, scale to a larger shard count that can hold all of the stored keys. For more information about increasing the number of shards for your instance, see Scale the shard count.
You scaled to a smaller node type that doesn't have the capacity to hold all of the data that Memorystore for Valkey stores in the original node type. If this occurs, then Memorystore for Valkey provides you with a recommended node type to scale down vertically.
You updated the node type for your instance, and either you overwrite the default settings for the original node type or these settings are overwritten already. However, the new, scaled node type either doesn't support the values for the default settings or the settings aren't valid for the node type. If this occurs, then Memorystore for Valkey returns an error. To resolve this, modify the settings manually so that they're valid for the new node type.
You scaled your instance during a period of high write pressure (for example, during load testing). To resolve this, scale during periods of low instance traffic.

Best practices

To help you scale the capacity for your instance, and to increase the speed and reliability of scaling your instance, scale it during low periods of traffic, whenever you can. To learn how to monitor instance traffic, see Monitor instances.