Redo Mechanism (Continued)

Now what happens if LGWR has written on all the redo log groups one by one? It’s a solid question. The answer is that when LGWR has written on all the redo log groups it comes back to the first one. Now here are two considerations. First is that if your database is running in No Archive Log mode than LGWR will start over writing the groups starting from the first one. But if your database is running in Archived Log mode than LGWR will not over write the log group until ARCH (an optional background process) has made a copy of that redo log group. This offline copy of online redo log is called Archived Redo Log. The LSN number of the redo group of which the copy is created is also stored in the copy. By default oracle database runs in No Archived Log mode but it is highly recommended that you must change it to Archived Log mode.

Here are some commands that you can use in dealing with redo log files. First of all if you want to see the information about redo log groups then you can check V$log view. You will see the number of groups, and members in each group and also some other information. You can’t see this information on operating system level; because this is hidden from operating system. If you want to see the exact location of the members of the redo log groups then open the V$logfile view.

If you want to add a redo log group than use the command:

SQL>alter database add logfile group 4 (‘c: \oracle\oradata\dbase\log41.log’) size 1M;

The second part is the path of the member of the group you created. Name of the member can be anything.

If you want to add a member files in the existing group than use the command:

SQL>alter database add logfile member ‘C: \oracle\oradata\dbase\log42.log’ to group 4;

Remember here you will not mention the size of the file because it is same as of other group members; in this case it will be 1M.

If you want to drop a member files from a group than use the command:

SQL>alter database drop logfile member ‘C: \oracle\oradata\dbase\log42.log’;

If you want to drop a group than use the command:

SQL>alter database drop logfile group 4;

If you want to explicitly switch log at any instant of time than use the following command. Remember that when a log switch occurs than event of check point occurs. And when check point occurs the CKPT process is activated and what it does is already been discussed. This command is useful for synchronization of data and is recommended before and after hot backups.

SQL>alter system switch logfile;

If you want check point to occur than use the following command. The same thing will also happen here except there will be no log switch.

SQL>alter system checkpoint;

Be careful when you are performing all these operations your database should be in mount state not in open.