Work with Oracle database redo log files

Database Migration Service leverages the Oracle LogMiner API, which is part of Oracle Database, to query archived redo log files. These files contain information about the history of activity on a database. Each Oracle database has a set of online redo log files. All transaction records on the database are recorded in the files.

When the current redo log file is rotated (or switched), the archive process copies this file into an archive storage. Meanwhile, the database promotes another file to serve as the current file.

When Database Migration Service uses the Oracle LogMiner API, it doesn't access the online redo log files but only works with the archived log files. Accessing archived redo log files inherently adds some latency to the migration process. This page describes suggested configuration for your Oracle source databases to control the latency impact.

Set configuration parameters for Oracle redo log files

This design has profound implications on Database Migration Service's potential latency. If Oracle redo log files are switched frequently or kept to a smaller size (for example, < 256MB), Database Migration Service can replicate changes faster.

There are configuration parameters that you can set to control the log file rotation frequency:

Size: Online redo log files have a minimum size of 4 MB, and the default size is dependent on your operating system. You can modify the size of the log files by creating new online log files and dropping the older log files.

To find the size of the online redo log files, run the following query:
```
SELECT GROUP#, STATUS, BYTES/1024/1024 MB FROM V$LOG
```
For self-hosted Oracle, use this guide. For Amazon RDS Oracle, use this guide.
Time: The ARCHIVE_LAG_TARGET parameter provides an upper limit of how long (in seconds) the current log of the primary database can span.
This isn't the exact log switch time, because it takes into account how long it will take to archive the log. The default value is 0 (no upper bound), and a reasonable value of 1800 (or 30 minutes) or less is suggested.

You can use the following commands to set the ARCHIVE_LAG_TARGET parameter, either during initialization or while the database is up:
- SHOW PARAMETER ARCHIVE_LAG_TARGET; This command displays how many seconds it will take for the current log to span.
- ALTER SYSTEM SET ARCHIVE_LAG_TARGET = number-of-seconds; Use this command to change the upper limit.
  For example, to set the upper limit to 10 minutes (or 600 seconds), enter ALTER SYSTEM SET ARCHIVE_LAG_TARGET = 600;