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When you multiplex redo log files, you are maintaining identical copies of the same files. Let s say you create two copies of a redo log file. You need to create a redo log group to contain these two identical files, which are called members of the group. At any given time, the LGWR process will write to a single group of redo log files, and the members of that group are then said to be active. Here are some basic conditions for Oracle redo log groups: All groups must contain an identical number of redo log files (members). All redo log files in a group must be identically sized.

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The where script makes the actual connections to the remote machines. The nohup command that calls the where xterm window lets us avoid interrupting any of the child windows when the parent window is closed. This completes the first script in the pair.

Although you can put both members of an online redo log group on the same physical disk, it s smart to locate them on different disks so one identical member can survive a disk crash that involves another member of the same group. Oracle will continue to write to the surviving members of the online redo log group when one member is not writable (perhaps due to a problem involving the disk drive).

You can create online redo log groups when you create a database for the first time. Here s an example showing just the redo log creation statement as part of the database creation process. Note that the three redo log groups each have only a single member they are not multiplexed at this point. SQL> CREATE DATABASE . . . LOGFILE GROUP 1 ('/u01/app/oracle/nicko/redo01.log') SIZE 100M, GROUP 2 ('/u01/app/oracle/nicko/redo02.log') SIZE 100M, GROUP 3 ('/u01/app/oracle/nicko/redo03.log') SIZE 100M, . . . Database created. SQL>

Although you need a minimum of two online redo log groups, the ideal number of online redo log groups for your database can only be worked out from the transaction activity in your database.

Start with two or three online redo log groups and monitor your alert log for any redo log errors. If the alert Tip log frequently shows that the log writer was waiting to write to an online redo log, you have to increase the number of redo groups.

The where.sh script is run within the original small window appearing on the user s X display. The only output this small window gives is a Node: prompt for user input. The script determines the type of connection and the output color of the new window that contains the remote system connection. First the script defines a few variables. The stty commands set up the Backspace and Ctrl+C (interrupt) key sequences for use in the small where window.

The following statement, which uses the ADD LOGFILE GROUP syntax, adds a new group of redo logs to your database. Note that this new redo log group is duplexed; two redo log files are being created in the group, not one: SQL> ALTER DATABASE ADD LOGFILE GROUP 4 ('/u01/app/oracle/nicko/log4a.rdo', ('/u01/app/oracle/nicko/log4b.rdo') SIZE 500M; Database altered. SQL> In the example in the previous section, we created three online log groups, but each of them had only a single member. To duplex those groups to provide additional safety, we need to add a member to each group. To add a single member to an existing group, you use the ADD LOGFILE MEMBER statement: SQL> ALTER DATABASE ADD LOGFILE MEMBER '/u01/app/oracle/nicko/log1b.rdo' TO GROUP 2; Database altered. SQL> Note that we didn t have to specify a size for the new redo log member being added to group 2 the new member will simply be sized the same as the existing members of the group.