Physical Structure

Physical Database Structures
The Oracle database consists of several types of operating system files, which it uses to manage itself. Although you call these files physical files to distinguish them from the logical entities they contain, understand that from an operating system point of view, even these files are not really physical, but rather logical components of the physical disk storage supported by the operating system.
Oracle Data Files
Data files are the way Oracle allocates space to the tablespaces that make up the logical database. The data files are the way Oracle maps the logical tablespaces to the physical disks. Each tablespace consists of one or more data files, which in turn belong to an operating system file system. Therefore, the total space allocated to all the data files in an Oracle database will give you the total size of that database.
When you create your database, your system administrator assigns you a certain amount of disk space based on your initial database sizing estimates. All you have at this point are the mount points of the various disks you are assigned (e.g., /prod01, /prod02, /prod03, and so on). You then need to create your directory structure on the various files. After you install your software and create the Oracle administrative directories, you can use the remaining file system space for storing database objects such as tables and indexes. The Oracle data files make up the largest part of the physical storage of your database. However, other files are also part of the Oracle physical storage, as you'll see in the following sections.
The Control File
Control files are key files that the Oracle DBMS maintains about the state of the database. They are a bit like a database within a database, and they include, for example, the names and locations of the data files and the all-important System Change Number (SCN), which indicates the most recent version of committed changes in the database. Control files are key to the functioning of the database, and recovery is difficult without access to an up-to-date control file. Oracle creates the control files during the initial database creation process.
Due to its obvious importance, Oracle recommends that you keep multiple copies of the control file. If you lose all your control files, you won't be able to bring up your database without re-creating the control file using special commands. You specify the name, location, and the number of copies of the control file in the initialization file (init.ora). Oracle highly recommends that you provide for more than one control file in the initialization file. Here's a brief list of the key information captured by the Oracle control file:
Checkpoint progress records
Redo thread records
Log file records
Data file records
Tablespace records
Log file history records
Archived log records
Backup set and data file copy records
Control files are vital during the time the Oracle instance is operating and during database recovery. During the normal operation of the database, the control file is consulted periodically for necessary information on data files and other things. Note that the Oracle server process continually updates the control file during the operation of the database. During recovery, it's the control file that has the data file information necessary to bring the database up to a stable mode.
The Redo Log Files
The Oracle redo log files are vital during the recovery of a database. The redo log files record all the changes made to the database. In case you need to restore your database from a backup, you can recover the latest change made to the database from the redo log files.
Redo log files consist of redo records, which are groups of change vectors, each referring to a distinct atomic change made to a data block in the Oracle database. Note that a single transaction may have more than one redo record. If your database comes down without warning, the redo log helps you determine if all transactions were committed before the crash or if some were still incomplete. Initially, the contents of the log will be in the redo log buffer (a memory area), but they will be transferred to disk very quickly.
Oracle redo log files contain the following information regarding database changes made by transactions:
Indicators to let you know when the transaction started
Name of the transaction
Name of the data object (e.g., an application table) that was being updated
The "before image" of the transaction (i.e., the data as it was before the changes were made)
The "after image" of the transaction (i.e., the data after the transaction changed the data)
Commit indicators that let you know if and when the transaction completed