Introduction ============ Redo logs are essential to the recovery of your database. They record all the changes that are made to the database. They also ensure the integrity of your database. Archiving redo logs will help .. ensure a successful backup ORACLE has provided numerous views in version 7 that allow you to monitor your redo logs The following views are discussed: v $ log v $ log_history v $ logfile v $ loghist v $ database Oracle7 redo logs ==== =============
Redo logs are key components in the recovery of your database. Redo logs contain both the redo and undo changes needed to make your database consistent. When an user commits a transaction, ORACLE guarantees that the changes to the blocks are written to the redo logs. Furthermore, undo information needed to roll back uncommitted transactions are also stored in the redo logs / archive files. Thereby, during instance recovery, oracle will roll forward applying the redo and then roll back applying the undo. The result is a consistent version of the . database in oracle 7, there are numerous views that allow the user to familiarize themselves with redo logs Here are a listing of these views as well as some helpful hints in interpreting the information found in these views v $ log:.. log file information From control file. name null? type description ----------------------------------------- ------------------------- Group # NUMBER LOG GROUP NUMBER THREAD # NUMBER LOG THREAD NUMBER SEQUENCE # NUMBER LOG SEQUENCE NUMBER BYTES NUMBER SIZE OF THE LOG IN BYTES MEMBERS NUMBER NUMBER OF MEMBERS IN LOG GROUP ARCHIVED VARCHAR2 (3) ARCHIVE STATUS: T / F STATUS VARCHAR2 (8) STATUS OF THE REDO LOGS FIRST_CHANGE # NUMBER LOWEST SCN iN THE LOG FIRST_TIME VARCHAR2 (20) TIME OF THE FIRST SCN iN THE LOG There must be at least two groups with one member each in every database. We recommend to have at least two members in each group On Separate Disk Drives;
thereby, you will be preventing the case where there is just one single point of redo failure. Each instance will have an associated thread and the combination of the thread # and the sequence # distinguishes a log file and an archive file. Each member within a GROUP SHOULD HAVE THE SIZE REDO LOGS But We Recommend That All Groups Have Identical Sizes. SCN (System Commit Number) Is The Key Internal Value That Determines At What State or "Time"
the database is at. Every time you do a select, you basically grab a consistent view of the database, a SCN. Thereby, a SCN is like a logical clock. When you recover, you are recovering to a specific time of the database. ARCHIVED: YES The content of the file has been archived For a newly added redo log file, the field is YES, regardless the archive mode NO otherwise STATUS:... UNUSED indicates the online redo log has not been written to This is the. state of a redo log that was just added or just after a RESETLOGS when it is not the current redo log. CURRENT indicates this is the current redo log. This implies that the redo log is active. The redo log could be open or closed. Active Indicate The Log Is Active But Is Not The Current Log. It is needed for crash recovery. IT . May be in use for block recovery It might or might not be archived INACTIVE indicate the log is no longer needed for crash recovery It may be in use for media recover It might or might not be archived v $ log_history:.... Archived LOG NAMES for All logs in the log history. Name Null?
TYPE DESCRIPTION -------------------------------------------------------------------------------- -------------- THREAD # NUMBER THREAD NUMBER OF ARCHIVED LOG SEQUENCE # NUMBER SEQUENCE NUMBER TIME VARCHAR2 (20) TIME OF THE LOWEST SCN IN THE LOG LOW_CHANGE # NUMBER LOWEST SCN HIGH_CHANGE # NUMBER HIGHEST SCN Archive_name varchar2 (257) Archive File Name V $ logfile: Information about all redo logs. Name null? Type description -------------- ------------ ---------------------------- Group # Number Redo log group identifier number status varcha2 (7) status of this log member member varchar2 (257 ) Redo log member name status: invalid -file is inaccessible Stale -file Contents Are INCOMPLETE DELETE -FILE IS NO LONGER Used Blank -file IS in use. V $ loghist: log history from the control file. Name Null? Type description -------------------- ---- ----- ---------------- Thread # Number log thread number sequence # Number log sequence number first_change # Number Lowest SCN The log first_time varcha2 (20) Time of the first scn the log Switch_change # Number SCN AT Which the log switch occurred;
One More Than THE HIGHEST SCN The Log. V $ Database: Database Information from The Control File. Name Null? Type Description ------------------------ ---- ------------------------- Name varchar2 (9) Name of the database created varcha2 (20) Creation Date log_mode varchar2 (12 ) ARCHIVE LOG MODE:. ARCHIVELOG / NOARCHIVELOG cHECKPOINT_CHANGE # NUMBER LAST SCN CHECKPOINTED ARCHIVE_CHANGE # NUMBER LAST SCN ARCHIVED In sqldba, you can get archival information by typing ARCHIVE LOG LIST Information about your redo logs are also shown Here is an example:.. SQLDBA > archive log list Database log mode NOARCHIVELOG Automatic archival DISABLED Archive destination /u04/oracle/6037p/dbs/arch.dbf Oldest online log sequence 248 Current log sequence 249 Database log mode I ndicates whether or not your database is in archive mode or not. Initially, it is set at database creation time using the CREATE DATABASE statement. Then you can change it while your database is in the mounted exclusive stage with the command ALTER DATABASE ARCHIVELOG / NOARCHIVELOG Automatic archival Indicates if you are automatically archiving (ENABLED) or if you are manually archiving (DISABLED). This is set in your init.ora with the log_archive_start parameter or you can manually adjust this with the command LOG ARCHIVE START / STOP ..
