If you are getting an error message as "There is not enough space on the disk to perform restore database operation."
-- This issue appears when rds restore starts, RDS makes an estimate about how much disk space will be required by the database that is going to be restored. If that estimate exceeds the amount of free space left on the disk, you will see this error.
Kindly note that as part of the restore process is trying to look for the data and log file size as was configured on the source and if the amount of space is not available on disk upfront its failing the restore process in the first place while trying perform the restore and also the SQL server restore process will not initiate the Storage Autoscaling.
For example: lets say the source database (On-prem\EC2\RDS sql server instance) is configured with 30 GB of data file and 30 GB of log file sizes and the used space inside the 30 GB data file is only 10 GB and used space inside the 30 GB log file is only 10 GB.The backup size of the database in this case would be only 20 GB(with compression this backup file size can further be reduced) out of actual 60 GB configured on the source database.During the restore of this 20 GB backup file on the RDS SQL server instance the process looks for the 60 GB of space on the underlying storage per the source data and log file configurations.In this case if the Target RDS database instance if it has 50 GB of underlying storage so restore process would fail as it requires 60 GB of storage space to proceed with the restore and it fails with the space issue.
Now I would suggest you to please check the actual database size on the source database using below script:
--Useful script to know the current size of database files and the free space.
USE [DBNAME] -- update your db name
GO
SELECT DB_NAME() AS DbName,
name AS FileName,
size/128.0 AS CurrentSizeMB,
size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS INT)/128.0 AS FreeSpaceMB
FROM sys.database_files;
If log is using the most of the storage space then you can try to shrink the log file and then again the backup and try to restore again.
