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When creating a new RDS MS SQL instance it requires a subnet group that spans multiple availability zones. I understand that this is different to Multi A-Z replication and that my instance will exist within a single availability region. So what is this actually doing when I'm adding this subnet group spanning different zones?
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Q. Can a subnet span Availability Zones? No. A subnet must reside within a single Availability Zone.
Amazon RDS requires at least two subnets in two different Availability Zones to support Multi-AZ DB instance deployments. This tutorial creates a Single-AZ deployment, but the requirement makes it easier to convert to a Multi-AZ DB instance deployment in the future. Choose Create VPC.
Other important benefits from having multi-AZ deployments include: No I/O delays during backups, as backups are taken from the standby instance. No interruptions to I/O when applying patches or performing upgrades for maintenance purposes. Increase in responsiveness when load balancing is used.
What would happen to an RDS (Relational Database Service) Multi-Availability Zone deployment if the primary DB instance fails? IP of the primary DB Instance is switched to the standby DB Instance. A new DB instance is created in the standby availability zone.
RDS uses the additional availability zone for storing backup data and logs for redundancy, the data for your instance is stored within the other availability zone.
More info can be found here:
https://aws.amazon.com/rds/faqs/
Q: Why does additional backup storage cost more than allocated DB Instance storage?
The storage provisioned to your DB Instance for your primary data is located within a single Availability Zone. When your database is backed up, the backup data (including transactions logs) is geo-redundantly replicated across multiple Availability Zones to provide even greater levels of data durability. The price for backup storage beyond your free allocation reflects this extra replication that occurs to maximize the durability of your critical backups.
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