Initialization Parameter Files

When you start the instance, an initialization parameter file is read. There are two types of parameter files. Server parameter file (SPFILE): This is the preferred type of initialization parameter file. It is a binary file that can be written to and read by the database server and must not be edited manually. It resides in the server on which the Oracle database is executing; it is persistent across shutdown and startup. The default name of this file, which is automatically sought at startup, is spfile<SID>.ora. Text initialization parameter file: This type of initialization parameter file can be read by the database server, but it is not written to by the server. The initialization parameter settings must be set and changed manually by using a text editor so that they are persistent across shutdown and startup. The default name of this file (which is automatically sought at startup if an SPFILE is not found) is init<SID>.ora. It is recommended that you create an SPFILE as a dynamic way to maintain initialization parameters. By using an SPFILE, you can store and manage your initialization parameters persistently in a server-side disk file.

Overview of Data Files

At the operating system level, Oracle Database stores database data in data files. Every database must have at least one data file.

Use of Data Files

Part I, "Oracle Relational Data Structures" explains the logical structures in which users store data, the most important of which are tables. Each nonpartitioned schema object and each partition of an object is stored in its own segment. For ease of administration, Oracle Database allocates space for user data in tablespaces, which like segments are logical storage structures. Each segment belongs to only one tablespace. For example, the data for a nonpartitioned table is stored in a single segment, which is in turn stored in one tablespace. Oracle Database physically stores tablespace data in data files. Tablespaces and data files are closely related, but have important differences:

Each tablespace consists of one or more data files, which conform to the operating system in which Oracle Database is running. The data for a database is collectively stored in the data files located in each tablespace of the database. A segment can span one or more data files, but it cannot span multiple tablespaces. A database must have the SYSTEM and SYSAUX tablespaces. Oracle Database automatically allocates the first data files of any database for the SYSTEM tablespace during database creation.

The SYSTEM tablespace contains the data dictionary, a set of tables that contains database metadata. Typically, a database also has an undo tablespace and a temporary tablespace (usually named TEMP). Figure shows the relationship between tablespaces, data files, and segments. Figure Data Files and Tablespaces

Permanent and Temporary Data Files

A permanent tablespace contains persistent schema objects. Objects in permanent tablespaces are stored in data files. A temporary tablespace contains schema objects only for the duration of a session. Locally managed temporary tablespaces have temporary files, which are special files designed to store data in hash, sort, and other operations. Temp files also store result set data when insufficient space exists in memory. Temp files are similar to permanent data files, with the following exceptions:

Permanent database objects such as tables are never stored in temp files. Temp files are always set to NOLOGGING mode, which means that they never have redo generated for them. Media recovery does not recognize temp files. You cannot make a temp file read-only. You cannot create a temp file with the ALTER DATABASE statement. When you create or resize temp files, they are not always guaranteed allocation of disk space for the file size specified. On file systems such as Linux and UNIX, temp files are created as sparse files. In this case, disk blocks are allocated not at file creation or resizing, but as the blocks are accessed for the first time. Caution: Sparse files enable fast temp file creation and resizing; however, the disk could run out of space later when the temp files are accessed.

Temp file information is shown in the data dictionary view DBA_TEMP_FILES and the dynamic performance view V$TEMPFILE, but not in DBA_DATA_FILES or the V$DATAFILE view.

Online and Offline Data Files

Every data file is either online (available) or offline (unavailable). You can alter the availability of individual data files or temp files by taking them offline or bringing them online. Offline data files cannot be accessed until they are brought back online. Administrators may take data files offline for many reasons, including performing offline backups, renaming a data file, or block corruption. The database takes a data file offline automatically if the database cannot write to it. Like a data file, a tablespace itself is offline or online. When you take a data file offline in an online tablespace, the tablespace itself remains online. You can make all data files of a tablespace temporarily unavailable by taking the tablespace itself offline

Data File Structure

Oracle Database creates a data file for a tablespace by allocating the specified amount of disk space plus the overhead for the data file header. The operating system under which Oracle Database runs is responsible for clearing old information and authorizations from a file before allocating it to the database. The data file header contains metadata about the data file such as its size and checkpoint SCN. Each header contains an absolute file number and a relative file number. The absolute file number uniquely identifies the data file within the database. The relative file number uniquely identifies a data file within a tablespace. When Oracle Database first creates a data file, the allocated disk space is formatted but contains no user data. However, the database reserves the space to hold the data for future segments of the associated tablespace. As the data grows in a tablespace, Oracle Database uses the free space in the data files to allocate extents for the segment.

Overview of Control Files

The database control file is a small binary file associated with only one database. Each database has one unique control file, although it may maintain identical copies of it.

Use of Control Files

The control file is the root file that Oracle Database uses to find database files and to manage the state of the database generally. A control file contains information such as the following:

The database name and database unique identifier (DBID) The time stamp of database creation Information about data files, online redo log files, and archived redo log files Tablespace information RMAN backups

The control file serves the following purposes:

It contains information about data files, online redo log files, and so on that are required to open the database. The control file tracks structural changes to the database. For example, when an administrator adds, renames, or drops a data file or online redo log file, the database updates the control file to reflect this change.

It contains metadata that must be accessible when the database is not open. For example, the control file contains information required to recover the database, including checkpoints. A checkpoint indicates the SCN in the redo stream where instance recovery would be required to begin. Every committed change before a checkpoint SCN is guaranteed to be saved on disk in the data files. At least every three seconds the checkpoint process records information in the control file about the checkpoint position in the online redo log.

Oracle Database reads and writes to the control file continuously during database use and must be available for writing whenever the database is open. For example, recovering a database involves reading from the control file the names of all the data files contained in the database. Other operations, such as adding a data file, update the information stored in the control file.

Multiple Control Files

Oracle Database enables multiple, identical control files to be open concurrently and written for the same database. By multiplexing a control file on different disks, the database can achieve redundancy and thereby avoid a single point of failure. Note: Oracle recommends that you maintain multiple control file copies, each on a different disk.

If a control file becomes unusable, then the database instance fails when it attempts to access the damaged control file. When other current control file copies exist, the database can be remounted and opened without media recovery. If all control files of a database are lost, however, then the instance fails and media recovery is required. Media recovery is not straightforward if an older backup of a control file must be used because a current copy is not available.

Control File Structure

Information about the database is stored in different sections of the control file. Each section is a set of records about an aspect of the database. For example, one section in the control file tracks data files and contains a set of records, one for each data file. Each section is stored in multiple logical control file blocks. Records can span blocks within a section. The control file contains the following types of records:

Circular reuse records These records contain noncritical information that is eligible to be overwritten if needed. When all available record slots are full, the database either expands the control file to make room for a new record or overwrites the oldest record. Examples include records about archived redo log files and RMAN backups.

Noncircular reuse records These records contain critical information that does not change often and cannot be overwritten. Examples of information include tablespaces, data files, online redo log files, and redo threads. Oracle Database never reuses these records unless the corresponding object is dropped from the tablespace.

You can query the dynamic performance views, also known as V$ views, to view the information stored in the control file. For example, you can query V$DATABASE to obtain the database name and DBID. However, only the database can modify the information in the control file. Reading and writing the control file blocks is different from reading and writing data blocks. For the control file, Oracle Database reads and writes directly from the disk to the Program Global Area. Each process allocates a certain amount of its PGA memory for control file blocks.

Overview of the Online Redo Log

The most crucial structure for recovery is the online redo log, which consists of two or more preallocated files that store changes to the database as they occur. The online redo log records changes to the data files.

Use of the Online Redo Log

The database maintains online redo log files to protect against data loss. Specifically, after an instance failure the online redo log files enable Oracle Database to recover committed data not yet written to the data files. Oracle Database writes every transaction synchronously to the redo log buffer, which is then written to the online redo logs. The contents of the log include uncommitted transactions, undo data, and schema and object management statements. Oracle Database uses the online redo log only for recovery. Redo log files are a useful source of historical information about database activity.

How Oracle Database Writes to the Online Redo Log

The online redo log for a database instance is called a redo thread. In single-instance configurations, only one instance accesses a database, so only one redo thread is present. In an Oracle Real Application Clusters (Oracle RAC) configuration, however, two or more instances concurrently access a database, with each instance having its own redo thread. A separate redo thread for each instance avoids contention for a single set of online redo log files. An online redo log consists of two or more online redo log files. Oracle Database requires a minimum of two files to guarantee that one is always available for writing while the other is being archived.

Online Redo Log Switches

Oracle Database uses only one online redo log file at a time to store records written from the redo log buffer. The online redo log file to which the log writer process is actively writing is called the current online redo log file. A log switch occurs when the database stops writing to one online redo log file and begins writing to another. Normally, a switch occurs when the current online redo log file is full and writing must continue. However, you can configure log switches to occur at regular intervals, regardless of whether the current online redo log file is filled, and force log switches manually. Log writer writes to online redo log files circularly. When log writer fills the last available online redo log file, the process writes to the first log file, restarting the cycle. Filled online redo log files are available for reuse depending on the archiving mode:

If archiving is disabled, which means that the database is in NOARCHIVELOG mode, then a filled online redo log file is available after the changes recorded in it have been checkpointed (written) to disk by database writer. If archiving is enabled, which means that the database is in Archivelog mode, then a filled online redo log file is available to log writer after the changes have been written to the data files and the file has been archived.

In some circumstances, log writer may be prevented from reusing an existing online redo log file. For example, an online redo log file may be active (required for instance recovery) rather than inactive (not required for instance recovery). Also, an online redo log file may be in the process of being cleared.
Multiple Copies of Online Redo Log Files

Oracle Database can automatically maintain two or more identical copies of the online redo log in separate locations. An online redo log group consists of an online redo log file and its redundant copies. Each identical copy is a member of the online redo log group. Each group is defined by a number, such as group 1, group 2, and so on. Maintaining multiple members of an online redo log group protects against the loss of the redo log. Ideally, the locations of the members should be on separate disks so that the failure of one disk does not cause the loss of the entire online redo log. In below, A_LOG1 and B_LOG1 are identical members of group 1, while A_LOG2 and B_LOG2 are identical members of group 2. Each member in a group must be the same size. LGWR writes concurrently to group 1 (members A_LOG1 and B_LOG1), then writes concurrently to group 2 (members A_LOG2 and B_LOG2), then writes to group 1, and so on. LGWR never writes concurrently to members of different groups. Figure: Multiple Copies of Online Redo Log Files

Structure of the Online Redo Log

Online redo log files contain redo records. A redo record is made up of a group of change vectors, each of which describes a change to a data block. For example, an update to a salary in the employees table generates a redo record that describes changes to the data segment block for the table, the undo segment data block, and the transaction table of the undo segments. The redo records have all relevant metadata for the change, including the following:

SCN and time stamp of the change Transaction ID of the transaction that generated the change SCN and time stamp when the transaction committed (if it committed) Type of operation that made the change Name and type of the modified data segment