In the last few days you saw how to
create a database and how to add datafiles and tablespaces to customize that
database for your own use. Remember, the instance is made up of the files used
by Oracle, the processes or threads, and the memory used by Oracle. The files
used by the database are made up of the following:
- The datafiles. These are the actual files that are used to store tablespaces.
- The redo log files (sometimes just known as the redo log). This is where all redo information is kept. Without the redo log files you would not be able to recover from a system failure.
- The control files. These files contain information vital to the Oracle database. The information that is used to tell the instance where the datafiles and log files reside is stored in the control file.
- The parameter file. This file contains tuning information that is used by Oracle at startup. This is commonly referred to as the init.ora file.
There are also the RDBMS binaries
and other files such as backup files, archive log files, and so on, but they
are not really part of the database itself, even though they are essential to
the RDBMS.
As you saw on Day 7,
"Administering Tablespaces," a tablespace can hold four different
types of segments:
- Data segment--Used to hold tables and clusters
- Index segment--Used to hold indexes
- Rollback segment--Special types of segments that are used to store undo information
- Temporary segment--Used for storing temporary data
Today you will look at the rollback
segments. Data segments, index segments, and temp-orary segments are covered on
Days 12-15. The rollback segment is not only important to the basic function of
the Oracle RDBMS, but it has performance implications as well.
Redo Log Files
The redo log files are used to store
redo information. Each time data is changed in the database, a log record is
written describing the change(s). With this information the database can be
recovered in the event of a system failure.
If a catastrophic system failure
occurs, such as a power failure, component failure, or similar occurrence, the
Oracle instance will be aborted. The instance will be cut off immediately or,
in the event of a disk failure, the instance might crash. If this occurs, all
changed data in the buffer cache will be lost; only changes that have been
written out to disk will be saved.
New Term: When Oracle is restarted, the information
in the redo log file will be used to reproduce changes that have been made to
the database, thus saving as much work as possible. All previously committed
transactions will be recovered; this is also known as being rolled forward.
All transactions that had modified data but had not been committed will be
backed out; this is known as rolling back.
The redo log file is necessary for
proper recovery. If this file is lost due to a disk failure, you will not be
able to recover in the event of a system failure; therefore, you must protect
the redo log file against this kind of failure. I recommend you use disk
mirroring or RAID-1 on all redo log files.
Because the redo log files are so
critical to the recoverability of the system, it is recommended that you do not
use a caching disk controller with write-caching unless that cache is backed up
with a battery. In the event of a power failure, you must make sure that no
redo information is lost. It is often recommended that write-caching not be
used at all on the redo log, but I feel that if you have a battery back-up your
risk is reduced.
WARNING: If you use a write-caching on the
controller that has the redo log files and it is not backed up with a battery,
you are in danger of losing data. In the event of a power failure, you will
lose redo information and might not be able to recover.
Thank you
Asraful
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