AWS RDS backups for single az instance are stored in an s3 bucket managed by AWS (not visible to user) as per documentation.
My question is for a single az instance, is the automated backup that is taken only stored in that specific AZ instance, or since it is stored in an s3 bucket managed by AWS, region-specific, not az specific?
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Answers
By default S3 is a global service. All data is replicated in >=3 AZ. Therefore, your backups are stored in multiple AZs.
Also, S3 provides different availability classes. For example, you can set for your S3 bucket a class "One Zone-IA" and then all data will be stored in one AZ. You should look into bucket settings to get know which class is turned on.
Also S3 provdies different classes for different objects in one bucket. Therefore, pay attention to backup-file settings in S3 too.
Link: https://aws.amazon.com/en/s3/storage-classes
The automated snapshots stored in S3 are replicated across 3 more more AZs. If you need more than that, you can look into the AWS Backup service. AWS Backup can do things like immutable backups in a protected vault, and cross-region replication of your backups.