22-01-2006

23-01-2006

Control file maintains stat and status of all the data files and redo log files.

When ever there are structural changes made to database then back of control file is provided by Oracle.

Oracle provides two types of back up for control file:

1. Binary Back up.
2. Trace back up.

Trace back up is used when we fail to take the back up of control file.

Trace back is sort of creating command for generating control file.

REDO Log File:

Information about every operation in Oracle i.e. complete history of changes made in Oracle is recorded in REDO log files. REDO log files are also called as journals of database.

Whenever user wants to change any data he uses “update” command. On receiving the request Oracle reads the data to be changed and stores the data in memory. In memory required changes are made and then information of every thing changed is stored in REDO log file. It is sort of Hot Back.

Oracle requires minimum of two REDO log files. As the database is created and users start using it, the REDO log file and another file called Log Switch file come into action.

Oracle recommends minimum two such files. Minimum size of these files is 50K.

Oracle can operate in two modes:

1. Archival
2. No Archival

When Oracle is working in archival mode it makes the copy of REDO log files internally and gives it a Log Sequence Number.

REDO log files which are being worked on are called ONLINE REDO LOG FILES where as archived log files are OFFLINE REDO LOG FILES.

Archived files are stored on hard disk.

There are three types of databases basically:

1. Test database
2. Production database
3. Development database

All production databases should take place in Archival mode. Remaining databases i.e. Test and Development databases should work in Non-Archival mode.

REDO group consists of mirror image of ONLINE REDO FILES. It is not possible to store mirror images of ONLINE REDO LOG FILES on remote server.

Check point is very important concept of Oracle. To keep the frequency check point tuned or rather synchronized, the file size of ONLINE REDO LOG FILES should be same though not necessary.

Oracle has outstanding feature of allowing back up archived files on remote servers. That was the reason why NASDAQ was in a position to recover the data despite of World Trade Centre attack.