Database Structure and Space Management Overview

Figure 1–1 Databases, Tablespaces, and Datafiles

Database

This figure illustrates the following:

Each database is logically divided into one or more tablespaces.

One or more datafiles are explicitly created for each tablespace to physically store the data of all logical structures in a tablespace.

The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace. (The SYSTEM tablespace has 2 megabit (Mb) storage capacity, and USERS tablespace has 4 Mb).

The combined storage capacity of a database’s tablespaces is the total storage capacity of the database (6 Mb).

Online and Offline Tablespaces A tablespace can be online (accessible) or offline (not accessible). A tablespace is generally online, so that users can access the information in the tablespace. However, sometimes a tablespace is taken offline to make a portion of the database unavailable while allowing normal access to the remainder of the database. This makes many administrative tasks easier to perform.

Physical Database Structures

The following sections explain the physical database structures of an Oracle database, including datafiles, redo log files, and control files.

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Datafiles

Every Oracle database has one or more physical datafiles. The datafiles contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the datafiles allocated for a database.

The characteristics of datafiles are:

A datafile can be associated with only one database.

Datafiles can have certain characteristics set to let them automatically extend when the database runs out of space.

One or more datafiles form a logical unit of database storage called a tablespace, as discussed earlier in this chapter.

Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle. For example, assume that a user wants to access some data in a table of a database. If the requested information is not already in the memory cache for the database, then it is read from the appropriate datafiles and stored in memory.

Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once, as determined by the database writer process (DBWn) background process.

See Also: "Memory Structure and Processes Overview" on

page 1-21 for more information about Oracle’s memory and process structures

Redo Log Files

Every Oracle database has a set of two or more redo log files. The set of redo log files is collectively known as the redo log for the database. A redo log is made up of redo entries (also called redo records).

The primary function of the redo log is to record all changes made to data. If a failure prevents modified data from being permanently written to the datafiles, then the changes can be obtained from the redo log, so work is never lost.

To protect against a failure involving the redo log itself, Oracle allows a multiplexed redo log so that two or more copies of the redo log can be maintained on different disks.

The information in a redo log file is used only to recover the database from a system or media failure that prevents database data from being written to the datafiles. For

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Database Structure and Space Management Overview

example, if an unexpected power outage terminates database operation, then data in memory cannot be written to the datafiles, and the data is lost. However, lost data can be recovered when the database is opened, after power is restored. By applying the information in the most recent redo log files to the database datafiles, Oracle restores the database to the time at which the power failure occurred.

The process of applying the redo log during a recovery operation is called rolling forward.

See Also: "Database Backup and Recovery Overview" on page 1-50 for more information about redo log files

Control Files

Every Oracle database has a control file. A control file contains entries that specify the physical structure of the database. For example, it contains the following information:

Like the redo log, Oracle lets the control file be multiplexed for protection of the control file.

Use of Control Files Every time an instance of an Oracle database is started, its control file identifies the database and redo log files that must be opened for database operation to proceed. If the physical makeup of the database is altered (for example, if a new datafile or redo log file is created), then the control file is automatically modified by Oracle to reflect the change. A control file is also used in database recovery.

See Also: "Database Backup and Recovery Overview" on page 1-50 for more information about the use of control files in database recovery

Data Utilities

The three utilities for moving a subset of an Oracle database from one database to another are Export, Import, and SQL*Loader.

Export Utility The Export utility transfers data objects between Oracle databases, even if they reside on platforms with different hardware and software

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configurations. Export extracts the object definitions and table data from an Oracle database and stores them in an Oracle binary-format Export dump file typically located on disk or tape.

Such files can then be copied using file transfer protocol (FTP) or physically transported (in the case of tape) to a different site. They can be used with the Import utility to transfer data between databases that are on machines not connected through a network or as backups in addition to normal backup procedures.

When you run Export against an Oracle database, it extracts objects, such as tables, followed by their related objects, and then writes them to the Export dump file.

Import Utility The Import utility inserts the data objects extracted from one Oracle database by the Export utility into another Oracle database. Export dump files can be read only by Import. Import reads the object definitions and table data that the Export utility extracted from an Oracle database.

The Export and Import utilities can also facilitate certain aspects of Oracle

Advanced Replication functionality, such as offline instantiation.

See Also: Oracle9i Replication

SQL*Loader Utility Export dump files can be read only by the Oracle Import utility. If you need to read load data from ASCII fixed-format or delimited files, you can use the SQL*Loader utility. SQL*Loader loads data from external files into tables in an Oracle database. SQL*Loader accepts input data in a variety of formats, performs filtering (selectively loading records based on their data values), and loads data into multiple Oracle database tables during the same load session.

See Also: Oracle9i Database Utilities for detailed information about

Export, Import, and SQL*Loader

Data Dictionary Overview

Each Oracle database has a data dictionary. An Oracle data dictionary is a set of tables and views that are used as a read-only reference about the database. For example, a data dictionary stores information about both the logical and physical structure of the database. A data dictionary also stores the following information:

The valid users of an Oracle database

Information about integrity constraints defined for tables in the database

The amount of space allocated for a schema object and how much of it is in use

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