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Lesson 13

Oracle Database Architecture Conclusion

The following is what was discussed in this module. An Oracle database consists of the following types of files:
  1. An initialization file: Points to the control files, and contains a number of parameters affecting database operation.
  2. Control files: Point to the other database files, and contains information crucial to the operation of the database.
  3. Datafiles: Hold your table and index data.
  4. Log files: Keep a record of changes that have been made, and transactions that have been committed, for use when recovering the database.
  5. Archived log files: Are copies of the redo log files and enable point-in-time recovery of a database.
You have also learned about tablespaces, that they are a logical construct sitting between schema objects such as tables, and thephysical datafiles used to store those objects. Tablespaces provide you with flexibility in how your data is physically stored on disk.
Finally, you saw in detail how Oracle writes a record of database changes to the redo log. You saw that Oracle uses redo log files over and over, in a circular fashion, and you saw how Oracle archives those files when a database is running in archive log mode.

Oracle Database Architecture Glossary

This module introduced you to the following terms:
  1. archive log mode:A mode of the database that enables the archiving of the online redo log.
  2. archive log file: An archived redo log file in Oracle is a copy of an online redo log file that has been saved to an offline location. The online redo log files are used to record all changes made to the database, and the archived redo log files are used to recover the database in the event of a failure.
  3. background processes: A background process is a computer process that runs behind the scenes and without user intervention. Typical tasks for these processes include logging, system monitoring, scheduling, and user notification.
  4. checkpoint: 1. A data structure that marks the checkpoint position, which is the SCN in the redo thread where instance recovery must begin. Checkpoints are recorded in the control file and each data file header, and are a crucial element of recovery. 2. The writing of dirty data blocks in the database buffer cache to disk. The database writer (DBW) process writes blocks to disk to synchronize the buffer cache with the data files.
  5. commit: Action that ends a database transaction and makes permanent all changes performed in the transaction.
  6. controllers: Usually refers to disk controllers. A disk controller is a component (usually a circuit board card) that manages one or more disks.
  7. database buffer cache: The portion of the system global area (SGA) that holds copies of data blocks. All client processes concurrently connected to the database instance share access to the buffer cache.
  8. instance: The term instance refers to a set of memory structures and background processes that operate against an Oracle database. Database users (programs that you run) communicate with an Oracle instance (background processes), and the instance does the actual work of reading and writing data to and from the database files. Using Oracle Parallel Server option, it is possible to have multiple instances simultaneously operating against a single database.
  9. log switch: The point at which the log writer process (LGWR) stops writing to the active redo log file and switches to the next available redo log file. LGWR switches when either the active redo log file is filled with redo records or a switch is manually initiated.
  10. multiplex: By multiplexing a control file on different disks, the database can achieve redundancy and thereby avoid a single point of failure.
  11. redo log group: A group of redo log files that Oracle treats as a unit. Oracle writes the same information to each file, thus implementing a form of mirroring using software rather than hardware.
  12. redo log member: One file in a group of redo log files.
  13. redo log file: A file that contains part of a databases redo log
  14. system tablespace: The tablespace Oracle uses for the data dictionary. Oracle treats this differently from other tablespaces. It is the one tablespace created by the CREATE DATABASE statement. Every database must have one, and it can never be taken offline.
  15. storage parameters: Values that control the way in which Oracle allocates disk space for a given table, view, or other database object. Storage parameters allow you to control the size of an object's initial extent and subsequent extents, and the maximum number of extents.
  16. tablespace: A tablespace is a storage location where the actual data underlying database objects can be kept. It provides a layer of abstraction between physical and logical data, and serves to allocate storage for all DBMS managed segments.
Now that you are familiar with the file types used in an Oracle database, we will turn our attention in the next module to the various Oracle server processes that operate on those files.

Oracle Basic Architecture Terms and Definitions

DBA_DATA_FILESReturns information about data files
Initialization fileA text file containing parameters that affect the operation of the database
Control filePoints to the other database files
db_block_sizeControls the size of a physical database block
TablespaceConsists of one or more datafiles
V$LOGFILEReturns information about redo log members
Redo log memberA file that contains a record of changes made to the database
DatafileContains table and index data
Redo log groupA group of log files, where the same information is written to each
Archive log destinationThe location to which redo log files are copied by the archive process
db_block_buffersControls the amount of memory reserved for buffering database blocks
Archive log fileIs a redo log file that has been copied to another location for use in database recovery