The final components of the Oracle architecture are the physical files where our information resided on disk. Oracle has several types for data files, each for a different purpose:

* Database datafiles

* Control files

* Online redo logs

* Parameter files

* Other database related files

Let?s look at each of these physical files in a bit more detail.

Database datafiles

Database datafiles are physical files stored on disk. These files are used to store data on disk. Database datafiles are only written to by the DBWR processes that we introduced you to earlier (there is an exception or two to this statement, but for now, assume that this point this true).

These database datafiles are associated with Oracle ?tablespaces?, which are ?logical? containers for tables and indexes. 

Control files

The Control File of the database is a binary file that contains a great deal of database information. The control file contains the database name, data about the database log files. Oracle cannot function without valid control files.

Because the control file is so important, Oracle allows you to maintain duplicate copies of the control file. When you have more than one control file, then you are said to be multiplexing your control files. It is a good practice to put these multiple copies on different disks to protect the control file. Later in this book, we will demonstrate how to do this.

Online redo logs concepts

Think of the online redo logs like a tape recorder that records every change in the Oracle database. As changes occur, they are regularly recorded in the online redo logs, just like you might record a movie on your VCR.

In the event that a disk crashes, you may have to replace the disk and restore the disk data from a backup tape.  If this backup tape was several days ago, you have lost a lot of data.

Fortunately, Oracle can ?replay? the saved transactions in the online redo logs, and re-apply lost transactions back into the database. Many times, this means that Oracle can recover from a crash without the DBA having to do anything other than just telling the database to startup. We will cover the topic of online redo log files and how they relate to database recovery in later chapters of this book.

At a minimum, Oracle requires that you have two online redo logs assigned to the database. Oracle will write redo to the first log, and when the first log is full, Oracle will switch to the second log and write the same redo. Each of these individual online redo logs is known as an online redo log group.

The reason we call them groups is that there can be mirrored copies of the online redo log files in each group. Like control files, it?s a good idea to have multiplexed copies of the Online redo logs. Oracle allows you to define multiple copies of these files. Each copy is called a member. Each redo log group can have one or more members. We will discuss the online redo log files in more detail in later chapters.

Oracle Parameter file Concepts

The parameter file (sometimes called init.ora) contains configuration information for the database to use at startup time. The parameter file you configure how much RAM the database is going to use, where to find the control files, where to write trace files, and a whole host of other information. In most cases the database will not start without a parameter file. Oracle allows you to have a manual parameter file (called a PFILE) or a server-side parameter file (called a SPFILE).

Other Database Related Files

When working with the Oracle database you will be introduced to a number of different kinds of files. The following table is a list of the most common files and their general purpose. As we proceed through the chapters of this book, you will be introduced to many of these files in more detail.

* Oracle Trace Files - Created by Oracle in a number of different situations. These can be created as a result of a database crash, a session failure, an unexpected but non-fatal database failure, or based on specific user operational commands.

* Alert log - This is the general log file for each Oracle database.

Networking configuration files - These files are used to configure the different network components of the Oracle database. These include files such as tnsnames.ora and listener.ora.

* Oracle Database Software Binaries - The Oracle Database software includes the basic programs that allow the database to function.

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