11 Physical Storage Structures

Several mechanisms are available for allocating and managing the storage of these files. The most common mechanisms include:

• Oracle Automatic Storage Management (Oracle ASM)

  Oracle ASM includes a file system designed exclusively for use by Oracle Database. "Oracle Automatic Storage Management (Oracle ASM)" describes Oracle ASM.

• Operating system file system

  Most Oracle databases store files in a file system, which is a data structure built inside a contiguous disk address space. All operating systems have file managers that allocate and deallocate disk space into files within a file system.

  A file system enables disk space to be allocated to many files. Each file has a name and is made to appear as a contiguous address space to applications such as Oracle Database. The database can create, read, write, resize, and delete files.

  A file system is commonly built on top of a logical volume constructed by a software package called a logical volume manager (LVM). The LVM enables pieces of multiple physical disks to combine into a single contiguous address space that appears as one disk to higher layers of software.

• Cluster file system

  A cluster file system is a distributed file system that is a cluster of servers that collaborate to provide high performance service to their clients. In an Oracle RAC environment, a cluster file system makes shared storage appear as a file system shared by many computers in a clustered environment. With a cluster file system, the failure of a computer in the cluster does not make the file system unavailable. In an operating system file system, however, if a computer sharing files through NFS or other means fails, then the file system is unavailable.

A database employs a combination of the preceding storage mechanisms. For example, a database could store the control files and online redo log files in a traditional file system, some user data files on raw partitions, the remaining data files in Oracle ASM, and archived the redo log files to a cluster file system.

Oracle ASM is a high-performance, ease-of-management storage solution for Oracle Database files. Oracle ASM is a volume manager and provides a file system designed exclusively for use by the database.

Oracle ASM provides several advantages over conventional file systems and storage managers, including the following:

• Simplifies storage-related tasks such as creating and laying out databases and managing disk space

• Distributes data across physical disks to eliminate hot spots and to provide uniform performance across the disks

• Rebalances data automatically after storage configuration changes

To use Oracle ASM, you allocate partitioned disks for Oracle Database with preferences for striping and mirroring. Oracle ASM manages the disk space, distributing the I/O load across all available resources to optimize performance while removing the need for manual I/O tuning. For example, you can increase the size of the disk for the database or move parts of the database to new devices without having to shut down the database.

Oracle Managed Files is a file naming strategy that enables you to specify operations in terms of database objects rather than file names. For example, you can create a tablespace without specifying the names of its data files. In this way, Oracle Managed Files eliminates the need for administrators to directly manage the operating system files in a database. Oracle ASM requires Oracle Managed Files.

With user-managed files, you directly manage the operating system files in the database. You make the decisions regarding file structure and naming. For example, when you create a tablespace you set the name and path of the tablespace data files.

Through initialization parameters, you specify the file system directory for a specific type of file. The Oracle Managed Files feature ensures that the database creates a unique file and deletes it when no longer needed. The database internally uses standard file system interfaces to create and delete files for data files and temp files, control files, and recovery-related files stored in the fast recovery area.

Oracle Managed Files does not eliminate existing functionality. You can create new files while manually administering old files. Thus, a database can have a mixture of Oracle Managed Files and user-managed files.

Oracle Database physically stores tablespace data in data files.

Each nonpartitioned schema object and each partition of an object is stored in its own segment, which belongs to only one tablespace. For example, the data for a nonpartitioned table is stored in a single segment, which in turn is stored in one tablespace. 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).

The following figure shows the relationship between tablespaces, data files, and segments.

Figure 11-4 Data Files and Tablespaces

Description of "Figure 11-4 Data Files and Tablespaces"

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 (temp 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.

  Note:

  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.

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. The database cannot access offline data files until they are brought online.

You may take data files offline for many reasons, including performing offline backups 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.

Starting in Oracle Database 12c, you can use the ALTER DATABASE MOVE DATAFILE statement to move an online data file from one physical file to another while the database is open and accessing the file. You can use this technique to achieve the following goals:

• Move a tablespace from one kind of storage to another

• Move data files that are accessed infrequently to lower cost storage

• Make a tablespace read-only and move its datafiles to write-once storage, such as a write once read many (WORM) drive

• Move a database into Oracle ASM

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, which uniquely identifies the data file within the database, and a relative file number, which 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.

The following figure illustrates the different types of space in a data file. Extents are either used, which means they contain segment data, or free, which means they are available for reuse. Over time, updates and deletions of objects within a tablespace can create pockets of empty space that individually are not large enough to be reused for new data. This type of empty space is called fragmented free space.

Figure 11-5 Space in a Data File

Description of "Figure 11-5 Space in a Data File"

Oracle Database uses the control file to locate 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 (see "Overview of Instance Recovery"). 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.

Oracle Database enables multiple, identical control files to be open concurrently and written to the same database. By multiplexing a control file on different disks, the database can achieve redundancy and thereby avoid a single point of failure.

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, then you can remount the database and open it without media recovery. If all control files of a database are lost, however, then the database 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.

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

  A circular reuse record contains 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

  A noncircular reuse record contains 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.

As explained in "Overview of the Dynamic Performance Views", 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 (PGA). Each process allocates a certain amount of its PGA memory for control file blocks.

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 that it has not yet written to the data files.

Server processes write every transaction synchronously to the redo log buffer, which the LGWR process then writes to the online redo log. Contents of the online redo log include uncommitted transactions, and schema and object management statements.

As the database makes changes to the undo segments, the database also writes these changes to the online redo logs. Consequently, the online redo log always contains the undo data for permanent objects. You can configure the database to store all undo data for temporary objects in a temporary undo segment, which saves space and improves performance, or allow the database to store both permanent and temporary undo data in the online redo log.

Oracle Database uses the online redo log only for recovery. However, administrators can query online redo log files through a SQL interface in the Oracle LogMiner utility (see "Oracle LogMiner"). Redo log files are a useful source of historical information about database activity.

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 file is always available for writing in case the other file is in the process of being cleared or archived.

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