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chore(kb/aws/rds): revise multi-az section

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Michele Cereda
2025-10-01 10:04:51 +02:00
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knowledge base/cloud computing/aws/rds.md
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@@ -15,6 +15,7 @@
1. [Export snapshots to S3](#export-snapshots-to-s3)
1. [Restore](#restore)
1. [Multi-AZ instances](#multi-az-instances)
1. [Converting instances between Multi-AZ and Single-AZ](#converting-instances-between-multi-az-and-single-az)
1. [Operations](#operations)
1. [PostgreSQL: reduce allocated storage by migrating using transportable databases](#postgresql-reduce-allocated-storage-by-migrating-using-transportable-databases)
1. [Stop instances](#stop-instances)
@@ -529,14 +530,9 @@ Refer the [Backup] section for what this means.
## Multi-AZ instances
Refer [Multi-AZ DB instance deployments for Amazon RDS] and
[When modifying a Multi-AZ RDS to Single Instance the AZ was changed!!].
Refer [Multi-AZ DB instance deployments for Amazon RDS].
RDS DB instances can be configured for high availability and failover support by using Multi-AZ deployments.
RDS uses several different technologies to provide failover support. It supports MariaDB, MySQL, Oracle, PostgreSQL,
and RDS Custom for SQL Server DB instances.<br/>
Microsoft SQL Server DB instances use SQL Server Database Mirroring or Always On Availability Groups.
DB instances can be configured for high availability and failover support by using Multi-AZ deployments.
RDS provisions and maintains a synchronous standby replica in a different AZ, which continuously syncs with the primary
DB instance.<br/>
@@ -550,6 +546,13 @@ system maintenance, and helps protect the database against DB instance failure a
Multi-AZ DB instance deployments have increased costs, and write and commit latency compared to Single-AZ deployments
due to the synchronous data replication to the standby replica.
A Single-AZ configuration deploys a DB instance and its EBS storage volumes in one AZ.<br/>
A Multi-AZ configuration deploys a DB instance and its EBS storage volumes across two or more AZs.
RDS uses several different technologies to provide failover support. It supports MariaDB, MySQL, Oracle, PostgreSQL,
and RDS Custom for SQL Server DB instances.<br/>
Microsoft SQL Server DB instances use SQL Server Database Mirroring or Always On Availability Groups.
If an infrastructure defect results in any outage of a Multi-AZ DB instance, RDS automatically switches to the standby
replica.<br/>
The failover typically takes 60 to 120 seconds, but it depends on the database activity and other conditions at the time
@@ -557,6 +560,25 @@ the primary DB instance became unavailable. Large transactions or a lengthy reco
time.<br/>
When the failover is complete, it can take additional time for the RDS console to reflect the new AZ.
For a Multi-AZ deployment, RDS creates DB snapshots and automated backups **from the secondary instance** during the
automatic backup window. This prevents the backup process from suspending I/O activity on the primary instance on all
engines but SQL Server.<br/>
In a Single-AZ deployment, the backup process does result in a brief I/O suspension that can last from a few seconds to
a few minutes. The amount of time depends on the size and class of the DB instance.
A Single-AZ instance is unavailable during a scaling operation.<br/>
For Multi-AZ deployments, RDS achieves minimal downtime during certain OS patches or scaling operations by:
1. Applying OS maintenance and scaling operations to the secondary instance first.
1. Promoting the secondary instance to primary, and demoting the old primary instance to secondary.
1. Performing maintenance or modifications on the old primary, now secondary instance.
### Converting instances between Multi-AZ and Single-AZ
Refer
[What happens when I change my RDS DB instance from a Single-AZ to a Multi-AZ deployment or a Multi-AZ to a Single-AZ deployment?]
and [When modifying a Multi-AZ RDS to Single Instance the AZ was changed!!].
One can convert existing Single-AZ DB instances to Multi-AZ deployments just by modifying the DB instance.<br/>
This process involves minimal to no downtime, but requires planning around storage and performance impacts if done on
active instances.
@@ -564,14 +586,16 @@ active instances.
During a Single-AZ to Multi-AZ conversion, RDS:
1. Takes a snapshot of the primary DB instance's EBS volumes.
1. Creates new volumes for the standby replica from that snapshot.
1. Creates new volumes for the standby replica from that snapshot in another AZ.
1. Turns on synchronous block-level replication between the volumes of the primary and standby replicas.
1. Creates the new standby replica instance in the AZ where the volumes were created, and attaches them to it.
One can convert existing Multi-AZ DB instances to Single-AZ deployments just by modifying the DB instance.<br/>
This process involves minimal to no downtime, but requires planning around storage and performance impacts if done on
active instances.
During a Multi-AZ to Single-AZ conversion, RDS typically keeps the instance in the AZ where the primary was located.
During a Multi-AZ to Single-AZ conversion, RDS typically keeps the instance in the AZ where the primary was located and
deletes only the secondary instance and volumes. The change does **not** typically affect the primary instance.
## Operations
@@ -1037,6 +1061,7 @@ or write workloads and exceeds the instance type quotas.
[transporting postgresql databases between db instances]: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/PostgreSQL.TransportableDB.html
[understanding postgresql roles and permissions]: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.PostgreSQL.CommonDBATasks.Roles.html
[viewing instance status]: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/accessing-monitoring.html
[What happens when I change my RDS DB instance from a Single-AZ to a Multi-AZ deployment or a Multi-AZ to a Single-AZ deployment?]: https://repost.aws/knowledge-center/rds-convert-single-az-multi-az
[what is aws database migration service?]: https://docs.aws.amazon.com/dms/latest/userguide/Welcome.html
[When modifying a Multi-AZ RDS to Single Instance the AZ was changed!!]: https://repost.aws/questions/QUmpYrb5etT0-de8woCv4rYQ/when-modifying-a-multi-az-rds-to-single-instance-the-az-was-changed
[Why is my Amazon RDS DB instance in the storage-optimization state for a long time?]: https://repost.aws/knowledge-center/rds-stuck-in-storage-optimization