Standby Redo Logs

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Up to this point we have learned how to configure the transport services on the primary to assure that the redo data is getting transmitted to the standby according to our wishes. What we haven't discussed is what is happening on the standby, where the redo is being received. In this section we will come to understand the process flow that occurs on the standby once the primary's redo data has arrived.

When the LGWR or ARC process on the primary initiates a connection with the standby, the standby listener responds by spawning a process called the remote file server (RFS). The RFS process will create a network connection with the processes on the primary and will sit waiting for data to arrive. Once data begins arriving from the primary, the RFS process will place it into either standby redo logs or archive redo logs. You should think of standby redo logs as exact twins to online redo logs, except for the fact that they are only active when a database is in the standby role. In essence, they are just a separate pool of redo logs. The RFS process will pick the first available standby redo log and begin placing changes into that log. When a log switch occurs on the primary, we switch standby redo logs and the RFS process will go to the next available standby redo log. The standby redo log that we were into prior to the log switch will be archived by the standby database and that archive will be applied by log apply services. Standby redo logs are wonderful things, and Oracle highly recommends that you use them. If you plan on setting one of the higher-level protection modes or plan on using real-time apply (covered in the 'Starting Managed Recovery' section), the use of standby redo logs is mandatory.

Some guidelines to follow when creating standby redo logs are

• The number of standby redo logs should be the same number as online redo logs plus one.

• The standby redo logs should be exactly the same size as your online redo logs.

• You should create standby redo logs on both the primary and standby to facilitate seamless role changes.

• In a RAC environment, all standby redo logs should be on a shared disk and may be thread-specific if desired.

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