When a user updated a particular table (for example 100 rows) the old value will be retained in the roll back segments(Oracle 8) and now it is Undo segment (oracle 9i). If the user issue a rollback command the old value will be taken from the rollback segment(that too if undo_retention parameter set properly in the parameter file).
Shadow set of disks save as a backup in the event of disk failure. In most Operating System if any disk failure occurs it automatically switchover to place of failed disk.
Improved performance because most OS support volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks.
LGWR process wirtes all change vectors from theredo log buffer to online redo log file sequentially.
Multiplexing Redo log file called Mirroing. ( Keeping multiple copies in different disks)
1. Edit init.ora file set control_files parameter with multiple location
2. shutdown immediate
3. copy control file to multiple locations & confirm from init.ora contol_files parameter
4. start the database.
5. run this query for changes confirmation - select name from v$controlfile;