Tuesday, October 9, 2007

Overview Of Physical Structure Of a Database

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



This section includes the following topics:

Datafiles
Control Files
Online Redo Log Files
Archived Redo Log Files
Parameter Files
Alert and Trace Log Files
Backup Files


Datafiles

Every Oracle database has one or more physical datafiles, which 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.



Datafiles have the characteristics:

A datafile can be associated with only one database.
Datafiles can be defined to extend automatically when they are full.
One or more datafiles form a logical unit of database storage called a tablespace.


Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle Database. For example, if a user wants to access some data in a table of a database, and 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 background process database writer process (DBWn).



Datafiles that are stored in temporary tablespaces are called tempfiles. Tempfiles are subject to some restrictions.



Control Files

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

Database name
Names and locations of datafiles and redo log files
Timestamp of database creation


Oracle Database can multiplex the control file, that is, simultaneously maintain a number of identical control file copies, to protect against a failure involving the control file.



Every time an instance of an Oracle database is started, its control file identifies the datafiles, tempfiles, 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 Database to reflect the change. A control file is also used in database recovery.



Online Redo Log Files

Every Oracle Database has a set of two or more online redo log files. These online redo log files, together with archived copies of redo log files, are collectively known as the redo log for the database. A redo log is made up of redo entries (also called redo records), which 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 Database lets you create a multiplexed redo log so that two or more copies of the redo log can be maintained on different disks.



Archived Redo Log Files

When online redo log files are written to disk, they become archived redo log files. Oracle recommends that you enable automatic archiving of the redo log. Oracle Database automatically archives redo log files when the database is in ARCHIVELOG mode.



Parameter Files

Parameter files contain a list of configuration parameters for that instance and database. Both parameter files (pfiles) and server parameter files (spfiles) let you store and manage your initialization parameters persistently in a server-side disk file. A server parameter file has these additional advantages:

The file is concurrently updated when some parameter values are changed in the active instance.
The file is centrally located for access by all instance in a Real Application Services database.
Oracle recommends that you create a server parameter file as a dynamic means of maintaining initialization parameters.



Alert and Trace Log Files

Each server and background process can write to an associated trace file. When an internal error is detected by a process, the process dumps information about the error to its trace file. Some of the information written to a trace file is intended for the database administrator, while other information is for Oracle Support Services. Trace file information is also used to tune applications and instances. The alert file, or alert log, is a special trace file. The alert log of a database is a chronological log of messages and errors.

The following features provide automation and assistance in the collection and interpretation of trace and alert file information:

The Automatic Diagnostic Repository (ADR) is a system-managed repository for storing and organizing trace files and other error diagnostic data. ADR provides a comprehensive view of all the critical errors encountered by the database and maintains all relevant data needed for problem diagnosis and eventual resolution. When the same type of incident occurs too frequently, ADR performs flood control to avoid excessive dumping of diagnostic information.
The Incident Packaging Service (IPS) extracts diagnostic and test case data associated with critical errors from the ADR and packages the data for transport to Oracle.


Backup Files

To restore a file is to replace it with a backup file. Typically, you restore a file when a media failure or user error has damaged or deleted the original file.

User-managed backup and recovery requires you to actually restore backup files before you can perform a trial recovery of the backups.

Server-managed backup and recovery manages the backup process, such as scheduling of backups, as well as the recovery process, such as applying the correct backup file when recovery is needed.

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