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Showing content from https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.Overview.Differences.html below:

Differences between read replicas for DB engines

Because Amazon RDS DB engines implement replication differently, there are several significant differences that you should know about.

Replicas for RDS for Db2 have the following features and behaviors:

• Replication method – Physical replication.

• Transaction logs purge – RDS for Db2 purges the logs from the primary DB instance when the following conditions have been met:

  • The logs are at least two hours old.

  • The archive log retention hours setting has passed.

  • RDS for Db2 successfully replicated the logs to all the replica DB instances.

  This applies to both same AWS Region DB instances and cross-Region DB instances. For information about setting archive log retention hours, see rdsadmin.set_archive_log_retention.

• Writable replicas – A Db2 replica is a physical copy, and Db2 doesn't allow for writes in a replica. You can promote the replica to make it writable. The promoted replica has the replicated data to the point when the request was made to promote it.

• Backups – Automatic backups and manual snapshots are supported on RDS for Db2 replicas.

• Parallel replication – Archive log data is always transmitted in parallel from the primary database to all of its replicas.

• Standby state – The primary use for standby replicas is cross-Region disaster recovery. For information, see Working with replicas for Amazon RDS for Db2.

Read replicas for RDS for MariaDB and RDS for MySQL have the following features and behaviors:

• Replication method – Logical replication.

• Transaction logs purge – RDS for MariaDB and RDS for MySQL keep any binary logs that haven't been applied.

• Writable replicas – You can enable the MariaDB or MySQL read replica to be writable.

• Backups – Automatic backups and manual snapshots are supported on RDS for MariaDB or RDS for MySQL read replicas.

• Parallel replication – All supported MariaDB and MySQL versions allow for parallel replication threads.

• Mounted state – Not supported.

Read replicas for RDS for Oracle have the following features and behaviors:

• Replication method – Physical replication.

• Transaction logs purge – If a primary DB instance has no cross-Region read replicas, Amazon RDS for Oracle keeps a minimum of two hours of transaction logs on the source DB instance. Logs are purged from the source DB instance after two hours or after the archive log retention hours setting has passed, whichever is longer. Logs are purged from the read replica after the archive log retention hours setting has passed only if they have been successfully applied to the database.

  In some cases, a primary DB instance might have one or more cross-Region read replicas. If so, Amazon RDS for Oracle keeps the transaction logs on the source DB instance until they have been transmitted and applied to all cross-Region read replicas.

  For information about setting archive log retention hours, see Retaining archived redo logs.

• Writable replicas – An Oracle read replica is a physical copy, and Oracle doesn't allow for writes in a read replica. You can promote the read replica to make it writable. The promoted read replica has the replicated data to the point when the request was made to promote it.

• Backups – Automatic backups and manual snapshots are supported on RDS for Oracle read replicas.

• Parallel replication – Redo log data is always transmitted in parallel from the primary database to all of its read replicas.

• Mounted state – The primary use for mounted replicas is cross-Region disaster recovery. An Active Data Guard license isn't required for mounted replicas. For more information, see Working with read replicas for Amazon RDS for Oracle.

Read replicas for RDS for PostgreSQL have the following features and behaviors:

• Replication method – Physical replication.

• Transaction logs purge – PostgreSQL has the parameter wal_keep_segments that dictates how many write ahead log (WAL) files are kept to provide data to the read replicas. The parameter value specifies the number of logs to keep.

• Writable replicas – A PostgreSQL read replica is a physical copy, and PostgreSQL doesn't allow for a read replica to be made writable.

• Backups – Manual snapshots are supported for RDS for PostgreSQL read replicas. Automated backups for read replicas are supported for RDS for PostgreSQL 14.1 and higher versions only. You can't turn on automated backups for PostgreSQL read replicas for RDS for PostgreSQL versions earlier than 14.1. For RDS for PostgreSQL 13 and earlier versions, create a snapshot from a read replica if you want a backup of it.

• Parallel replication – PostgreSQL has a single process handling replication.

• Mounted state – Not supported.

Read replicas for RDS for SQL Server have the following features and behaviors:

• Replication method – Physical replication.

• Transaction logs purge – The Virtual Log File (VLF) of the transaction log file on the primary replica can be truncated after it is no longer required for the secondary replicas.

  The VLF can only be marked as inactive when the log records have been hardened in the replicas. Regardless of how fast the disk subsystems are in the primary replica, the transaction log will keep the VLFs until the slowest replica has hardened it.

• Writable replicas – A SQL Server read replica is a physical copy and also doesn't allow for writes. You can promote the read replica to make it writable. The promoted read replica has the replicated data up to the point when the request was made to promote it.

• Backups – Automatic backups and manual snapshots aren't supported on RDS for SQL Server read replicas.

• Parallel replication – Redo log data is always transmitted in parallel from the primary database to all of its read replicas.

• Mounted state – Not supported.

Working with DB instance read replicas

Creating a read replica

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