Do I need data backup and restore when I increase Amazon OpenSearch data node volume size?

No, you do not need to perform data backup and restore when increasing the Amazon OpenSearch Service data node volume size. When you increase the volume size, your existing data will remain intact and available.

Amazon OpenSearch Service has recently introduced a feature that allows you to update cluster volume size, volume type, IOPS, and throughput without requiring a blue/green deployment for most cases. This means you can make changes to your EBS settings more easily and with minimal disruption to your cluster operations.

This new feature is available for OpenSearch Service domains using current generation instances, and when the volume size per data node is within 3 TiB. It ensures that there is minimal performance impact on your online traffic and avoids potential disruptions to your cluster operations that were previously associated with blue/green deployments.

However, it's important to note that there are still some scenarios where a blue/green deployment might be required, such as when decreasing storage volume size or making storage volume changes within short intervals. You can use the dry-run option to check whether your specific change will require a blue/green deployment.

While this new feature makes volume size increases more straightforward, it's always a good practice to have regular backups of your data regardless of whether you're making changes to your cluster configuration. Amazon OpenSearch Service offers automated and manual snapshot capabilities for data durability, which you should utilize as part of your overall data management strategy.
Sources
Amazon OpenSearch Service now lets you update cluster volume without blue/green
Community | AWS open source newsletter, #189
Open-Source Search Engine - Amazon OpenSearch Service Pricing - AWS