Maintaining Redo Log Files参考.pptVIP

Maintaining Redo Log Files Objectives After completing this lesson, you should be able to do the following: Explain the purpose of online redo log files Outline the structure of online redo log files Control log switches and checkpoints Multiplex and maintain online redo log files Manage online redo logs files with OMF Using Redo Log Files Redo log files record all changes made to data and provide a recovery mechanism from a system or media failure. Redo log files are organized into groups. An Oracle database requires at least two groups. Each redo log within a group is called a member. Structure of Redo Log Files How Redo Logs Work Redo logs are used in a cyclic fashion. When a redo log file is full, LGWR will move to the next log group. This is called a log switch Checkpoint operation also occurs Information is written to the control file Forcing Log Switches and Checkpoints Log switches can be forced using the ALTER SYSTEM SWITCH LOGFILE command. Checkpoints can be forced using: Setting FAST_START_MTTR_TARGET parameter ALTER SYSTEM CHECKPOINT command Adding Online Redo Log Groups Adding Online Redo Log Members Dropping Online Redo Log Groups Dropping Online Redo Log Members Clearing, Relocating, or RenamingOnline Redo Log Files Clearing online redo log files: Relocating or renaming online redo log files can be accomplished by adding new members and dropping old members. Online Redo Log Configuration Managing Online Redo Logs with OMF A complete group can be added with no file specification: If a group is dropped, all the corresponding OMF files are deleted at the OS level: Obtaining Group and Member Information Information about group and members can be obtained by querying the data dictionary. V$LOG V$LOGFILE Archived Redo Log Files Filled online redo log files can be archived. Two advantages exist to archiving redo logs: Recovery: A database backup, together with online and archived redo log files can guarantee recovery of all