RMAN Redo Log Files
These files hold all changes made to database objects. Changes
made to the database are kept in redo log files in
the form of redo records. These redo records are made of change
vectors. These change vectors hold information about changes made to
data blocks of the database. Redo log files are vital for the
database recovery process. When you backup datafiles and restore
them in case of a database crash, you come back to the time when the
backup was taken. By applying archived redo log files, you bring the
database to the time before the crash happened.
There are mainly three methods for backing up physical files of
the database: RMAN (Recovery Manager), user-managed and using
Enterprise Manager which invokes RMAN commands in the background.
RMANoriginally was introduced in
Oracle 8 and since then, many functions have been added. By
employing this tool, you can take a backup of the database according
to any criteria. Oracle recommends that you back up your database
using RMAN. While using RMAN, you mainly use the command line
interface. In Oracle 10g, by using Enterprise Manager you can easily
take a backup of the database.
The user-managed type of
means taking backup of your database and
recovering it using a combination of SQL*Plus and OS/3rd party
utility commands. This concept is covered broadly in Chapter 11.
Additionally, Oracle provides flashback technologies for the
recovery process. By using flashback commands, you can easily get
back the data, such as a dropped table or huge amount of rows
changed incorrectly by using simple commands instead of restoring
datafiles and recovering it by adding all archived redo log files
which consume a lot of time. See Chapter 12 for more information.
First of all, you should know that if recovery is mentioned, it
means that you should have archived redo log files or incremental
backup. By adding archived redo log files to the last backup taken,
you do recover. For that, Oracle strongly recommends you run your
database in archivelog mode As you know, when the current online
redo log file is filled, LGWR begins to write to the next redo log
file. Without running the database in archivelog mode, the filled
redo log file will not be archived and will be rewritten by LGWR on
further log switches in a circular fashion. Therefore, you will not
have any changed information on the database except those that were
in datafiles and you will lose your data.
This can be seen from Bobs example illustrated in Figure 1.1.
According to the figure, if Bob had been running the database in
noarchivelog mode, then he would not have archived redo log files
which keep information from January 1 until January 10. In the event
of a database crash, he would be able to restore the database from
the January 1 backup, but would not be able to recover it.
After understanding the importance of backup, see what type of
backups should be taken by Oracles utility:
RMAN. Generally, RMAN mainly has the following types of backup
methods:
- Consistent and inconsistent backups (open and closed)
- Whole and partial backups
- Full and incremental backups
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