Unfortunately, you cannot reduce the allocated storage size when restoring an RDS Oracle DB instance from a snapshot [1]. Amazon RDS has a fundamental limitation where you can't reduce the amount of storage when restoring a DB instance [1]. This means that using a snapshot as a workaround to reduce storage size will not work for your use case.

Storage Modification Constraints

When working with RDS snapshots and storage, you should be aware that storage can only be increased, not decreased. If you do need to increase allocated storage during a restore operation, it must be by at least 10 percent - attempting to increase the value by less than 10 percent will result in an error [1].

Understanding RDS Oracle Snapshots

RDS creates a storage volume snapshot of your entire DB instance, backing up the complete DB instance rather than just individual databases [2]. For Oracle databases specifically, when your DB instance contains a container database (CDB), the snapshot includes the root CDB and all pluggable databases (PDBs).

Alternative Considerations

Since you cannot reduce storage through snapshots, you would need to explore other approaches such as data cleanup, archiving unused data, or migrating to a new instance with optimized storage planning. Amazon RDS for Oracle does provide storage autoscaling that can automatically scale storage capacity in response to growing database workloads with zero downtime [3], which can help manage future storage growth more efficiently.

You can use Oracle Data Pump (expdp/impdp), other data migration tools(DMS,GoldenGate, STRIIM) to migrate data from current instance to new instance(with less storage) to reduce the storage size. This is the only feasible option.

References:

[1] Title: "Restoring to a DB instance - Amazon Relational Database Service" URL: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_RestoreFromSnapshot.html

[2] Title: "Creating a DB snapshot for a Single-AZ DB instance for Amazon RDS - Amazon Relational Database Service" URL: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_CreateSnapshot.html

[3] Title: "Storage capacity - AWS Prescriptive Guidance" URL: https://docs.aws.amazon.com/prescriptive-guidance/latest/replatform-oracle-database-options/storage.html

Unfortunately, using a snapshot to reduce the storage size of an RDS Oracle database will not work as intended. When restoring an Amazon RDS instance from a snapshot, the minimum allocated storage size for the new instance is determined by the maximum storage size ever used by the original instance. This means that even if your current usage is lower, you cannot restore to a smaller storage size if the original instance was ever larger.

To reduce the storage size of your RDS Oracle database, you'll need to take a different approach. Here are some options:

1. Export and import: Create a new RDS instance with the desired smaller storage size, then export the data from the original instance and import it into the new one. This process involves using database export tools to transfer the data.

2. Optimize current storage: Before considering a reduction, try to optimize your current storage usage. This can include:

   • Managing archive log retention
   • Cleaning up large trace files and Data Pump directories
   • Addressing database fragmentation
   • Reviewing and adjusting file size quotas

3. Reduce backup retention: If you haven't already, consider reducing your backup retention period to free up storage space used by backups.

4. Use storage autoscaling: While this doesn't reduce your current size, it can help manage costs by only increasing storage when necessary.

If after these steps you still need to reduce storage, the export and import method to a new, smaller instance is likely your best option. Be aware that this process may involve some downtime and careful planning to ensure data integrity.

Remember, it's crucial to monitor your storage usage regularly and set up CloudWatch alarms to notify you when free storage space reaches a specified threshold. This allows you to take proactive measures before hitting critical storage levels.
Sources
Reducing Allocated Storage for RDS Instance to Optimize Costs(Caused by Storage Autoscaling) | AWS re:Post
Oracle RDS running out of storage | AWS re:Post
RDS server instance storage breakdown | AWS re:Post

Hi, reducing storage space for RDS Oracle is not a straightforward process. Here are some options which you can use to reduce the storage.

Direct Reduction Amazon RDS does not allow direct downsizing of Oracle database storage. Once you allocate storage to an RDS instance, you cannot reduce it directly.

Indirect Method The only way to effectively reduce storage is to create a new, smaller instance with less storage and migrate your data: Create a new RDS instance with less storage, Export data from the original instance, Import data into the new instance, Switch your application to the new instance, Delete the old instance.

Storage Optimization: While you can't reduce allocated storage, you can optimize usage: Delete unnecessary data, Use table and index compression, Archive old data to S3, Clear recycle bin and temporary files.

Prevent Future Growth: Enable Storage Auto-scaling with appropriate maximum threshold Implement data archiving strategies Regular maintenance (rebuilding indexes, purging old data)

Monitoring: Use CloudWatch to monitor "FreeStorageSpace" metric and set up alerts

Please remember, while you can't directly shrink RDS Oracle instance, you can optimize its use and plan for better sizing in future deployments. Always ensure you have recent backups before performing any significant changes to your database.

You cannot reduce the allocated storage size of an Amazon RDS for Oracle instance by restoring from a snapshot. This is because RDS snapshots are block-level backups that capture the entire allocated storage of the database instance, not just the actual data in use. When restoring from a snapshot, AWS uses the metadata contained in it to recreate the instance with the same allocated storage as the original, and does not allow restoration to a smaller size. Even if your database only uses a fraction of the allocated space, the restored instance will provision the full original size.

This behavior is intentional and stems from how Amazon RDS is designed to ensure data consistency and avoid the risks associated with resizing EBS volumes to a smaller size. Currently, RDS does not support decreasing the allocated storage for any of its supported engines, including Oracle, PostgreSQL, MySQL, and SQL Server.

If you need to reduce the storage size of your Oracle RDS instance, the recommended workaround is to create a new RDS instance with the desired smaller storage size and manually migrate the data to it. This can be done using Oracle Data Pump (expdp/impdp), SQL Developer, or other data migration tools. You would first export the data from the original instance, create the necessary schemas and users on the new instance, and then import the data. Once the data has been validated, and the application endpoints have been updated, the original larger instance can be safely decommissioned.

Restoring a snapshot does not provide the flexibility to reduce storage because the snapshot includes the full allocated storage metadata. Therefore, careful planning of initial storage allocation and ongoing monitoring is important to avoid over-provisioning and unnecessary storage costs. If you're using features like autoscaling storage, be sure to monitor metrics such as FreeStorageSpace using Amazon CloudWatch to avoid unexpected storage growth.