The following descriptions summarize the three distinct modes of data 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. |