Databases Reference
In-Depth Information
To simplify operations in the event of a failure, we recommend that you always take a
backup after an unlogged operation if you cannot afford to lose the object created by
the operation or you cannot repeat the operation for some reason. In addition to using
the NOLOGGING keyword in certain commands, you can also mark a table or an entire
tablespace with the NOLOGGING attribute. This will suppress redo information for
all applicable operations on the table or for all tables in the tablespace.
Multiplexing redo logfiles
Oracle defines specific terminology to describe how it manages redo logs. Each Oracle
instance uses a
thread
of redo to record the changes it makes to the database. A thread
of redo is composed of redo log groups, which are composed of one or more redo log
members.
Logically, you can think of a redo log group as a single redo logfile. However, Oracle
allows you to specify multiple copies of a redo log to protect the all-important integrity
of the redo log. By creating multiple copies of each redo logfile, you protect the redo
logfile from disk failure and other types of disasters.
Figure 2-5
illustrates a thread of redo with groups and members. The figure shows two
members per group, with each redo log mirrored.
Figure 2-5. A thread of redo
