In some situations, a business cannot afford to lose data. In other situations, the availability of the database may be more important than the loss of data. Some applications require maximum database performance and can tolerate some small amount of data loss. The following descriptions summarize the three distinct modes of data protection.
- Maximum availability
- Maximum performance
- Maximum protection
Maximum protection This protection mode ensures that no data loss will occur if the primary database fails. To provide this level of protection, the redo data needed to recover each transaction must be written to both the local online redo log and to the standby redo log on at least one standby database before the transaction commits. To ensure data loss cannot occur, the primary database shuts down if a fault prevents it from writing its redo stream to the standby redo log of at least one transactionally consistent standby database.
Maximum availability This protection mode provides the highest level of data protection that is possible without compromising the availability of the primary database. Like maximum protection mode, a transaction will not commit until the redo needed to recover that transaction is written to the local online redo log and to the standby redo log of at least one transactionally consistent standby database. Unlike maximum protection mode, the primary database does not shut down if a fault prevents it from writing its redo stream to a remote standby redo log. Instead, the primary database operates in maximum performance mode until the fault is corrected, and all gaps in redo log files are resolved. When all gaps are resolved, the primary database automatically resumes operating in maximum availability mode.
This mode ensures that no data loss will occur if the primary database fails, but only if a second fault does not prevent a complete set of redo data from being sent from the primary database to at least one standby database.
Maximum performance This protection mode (the default) provides the highest level of data protection that is possible without affecting the performance of the primary database. This is accomplished by allowing a transaction to commit as soon as the redo data needed to recover that transaction is written to the local online redo log. The primary database’s redo data stream is also written to at least one standby database, but that redo stream is written asynchronously with respect to the transactions that create the redo data.
When network links with sufficient bandwidth are used, this mode provides a level of data protection that approaches that of maximum availability mode with minimal impact on primary database performance.
The maximum protection and maximum availability modes require that standby redo log files are configured on at least one standby database in the configuration. All three protection modes require that specific log transport attributes be specified on the LOG_ARCHIVE_DEST_n initialization parameter to send redo data to at least one standby database. See Section 5.6 for complete information about the data protection modes.
Source : docs.oracle.com