ArchiveLog Mode

When you run a database in ARCHIVELOG mode, you enable the archiving of the redo log. The database control file indicates that a group of filled redo log files cannot be reused by LGWR until the group is archived. A filled group becomes available for archiving immediately after a redo log switch occurs.

The archiving of filled groups has these advantages:

•  A database backup, together with online and archived redo log files, guarantees that you can recover all committed transactions in the event of an operating system or disk failure. 
• If you keep an archived log, you can use a backup taken while the database is open and in normal system use.
• You can keep a standby database current with its original database by continuously applying the original archived redo logs to the standby.

You can configure an instance to archive filled redo log files automatically, or you can archive manually. For convenience and efficiency, automatic archiving is usually best. 

If all databases in a distributed database operate in ARCHIVELOG mode, you can perform coordinated distributed database recovery. However, if any database in a distributed database is in NOARCHIVELOG mode, recovery of a global distributed database (to make all databases consistent) is limited by the last full backup of any database operating in NOARCHIVELOG mode

Related Post :- Setting the Initial Database Archiving Mode

Read More

Archived Redo Logs Files

What Is the Archived Redo Log?

Oracle Database lets you save filled groups of redo log files to one or more offline destinations, known collectively as the archived redo log, or more simply the archive log. The process of turning redo log files into archived redo log files is called archiving. This process is only possible if the database is running in ARCHIVELOG mode. You can choose automatic or manual archiving.

An archived redo log file is a copy of one of the filled members of a redo log group. It includes the redo entries and the unique log sequence number of the identical member of the redo log group. For example, if you are multiplexing your redo log, and if group 1 contains identical member files a_log1 and b_log1, then the archiver process (ARCn) will archive one of these member files. Should a_log1 become corrupted, then ARCn can still archive the identical b_log1. The archived redo log contains a copy of every group created since you enabled archiving.

When the database is running in ARCHIVELOG mode, the log writer process (LGWR) cannot reuse and hence overwrite a redo log group until it has been archived. The background process ARCn automates archiving operations when automatic archiving is enabled. The database starts multiple archiver processes as needed to ensure that the archiving of filled redo logs does not fall behind.

You can use archived redo logs to:

Recover a database

Update a standby database

Get information about the history of a database using the LogMiner utility


Related Post: NOARCHIVELOG and ARCHIVELOG Mode

Read More

Oracle Database

Oracle introduced the first commercial Relational Database Management System (RDBMS) almost 30 years ago. Oracle database consists of operating system files, also known as database files, that provide the actual physical storage for database information. The database files are used to ensure that the data is kept consistent and can be recovered in the event of a failure of the instance.

The general purpose of a database is to store and retrieve related information. An Oracle database has a logical and a physical structure. The physical structure of the database is the set of operating system files in the database. An Oracle database consists of three file types.

• Data files containing the actual data in the database
• Redo logs containing a record of changes made to the database to enable recovery of     the data in case of failures
• Control files containing information necessary to maintain and verify database integrity Other Key File Structures

     The Oracle server also uses other files that are not part of the database:  

• The parameter file defines the characteristics of an Oracle instance. For example, it contains parameters that size some of the memory structures in the SGA.
• The password file authenticates users privileged to start up and shut down an Oracle instance.
• Archived redo log files are offline copies of the redo log files that may be necessary to recover from media failures.

Related video:- Introduction to Oracle Database Administration - Oracle DBA

Read More

What is Database

A database is a collection of information that is organized so that it can easily be accessed, managed, and updated.Databases are designed to offer an organized mechanism for storing, managing and retrieving information. They do so through the use of tables.It allows organizations to conveniently develop databases for various applications by database administrators (DBAs) and other specialists.The term "database" may be narrowed to specify particular aspects of organized collection of data and may refer to the logical database, to physical database as data content in computer data storage or to many other database sub-definitions.The term database is correctly applied to the data and their supporting data structures, and not to the database management system (referred to by the acronym DBMS). The database data collection with DBMS is called a database system.The utilization of databases is now spread to such a wide degree that virtually every technology and product relies on databases and DBMSs for its development and commercialization, or even may have such embedded in it. Also, organizations and companies, from small to large, heavily depend on databases for their operations

Read More