Oracle Architecture

Instances and Databases (1/2)

Instances and Databases (2/2) Instance –Logical –Temporal Database –Physical –Forever An instance can connect to one and only one database.

The Component of a Database (1/21) Tablespace (1/2) –A logical structure –Each tablespace must consists of one or more datafiles, and each datafile can belong to only one tablespace.

The Component of a Database (2/21) Tablespace (2/2)

The Component of a Database (3/21) Three fundamental types of physical files in an Oracle database –Control files –Datafiles –Redo log files Oracle Managed Files (OMFs) (Oracle9i)

The Component of a Database (4/21)

The Component of a Database (5/21) Control files (1/3) –Contents A list of all the other files in the database The name of the database When the database was created The current state of the datafiles –Need recovery –Read-only If the database closed cleanly the last time it was shut down The time period covered by each archived redo log What backups have been performed for the database

The Component of a Database (6/21) Control files (2/3) –Control file parameters (INIT.ORA) MAXLOGFILES MAXLOGMEMBERS MAXLOGHISTORY MAXDATAFILES MAXINSTANCES

The Component of a Database (7/21) Control files (3/3) –Multiple control files At least two control files The CONTROL_FILE parameter in the INIT.ORA file

The Component of a Database (8/21) Datafiles (1/5) –Data Tables Indexes Data dictionary Rollback segments

The Component of a Database (9/21) Datafiles (2/5) –Oracle block Composed of operating system blocks The DB_BLOCK_SIZE parameter in INIT.ORA –2K to 32K –Oracle with Very Large Memory support »64KB –Prior to Oracle9i, a single block size for the entire database. –With Oracle9i »A default block size for the database, up to five nonstandard block sizes in the database. »A single block size for each tablespace, but block sizes can be mixed within a database. –OLTP vs. data warehouse

The Component of a Database (10/21) Datafiles (3/5)

The Component of a Database (11/21) Datafiles (4/5) –Datafile header The first block of each datafile. Contains critical information used to maintain the overall integrity of the database. –Checkpoint structure.

The Component of a Database (12/21) Datafiles (5/5) –Extent A set of data blocks that are contiguous within an Oracle datafile. –Segment An object that takes up space in an Oracle database.

The Component of a Database (13/21) Redo log files (1/9) –Redo log files store a “recording” of the changes made to the database as a result of transactions and internal Oracle activities. –The recording of the changes in the redo logs is vital because it plays back the changes that were lost when the failure occurs.

The Component of a Database (14/21) Redo log files (2/9) –Suppressing redo logging UNRECOVERABLE (Oracle7) NOLOGGING (Oracle 8)

The Component of a Database (15/21) Redo log files (3/9) –A thread of redo

The Component of a Database (16/21) Redo log files (4/9) –How Oracle uses the redo logs

The Component of a Database (17/21) Redo log files (5/9) –Naming conventions for redo logs

The Component of a Database (18/21) Redo log files (6/9) –Archived redo logs Two types of redo logs –Online redo logs –Archived redo logs Two modes with respect to archiving redo logs –NOARCHIVELOG –ARCHIVELOG

The Component of a Database (19/21) Redo log files (7/9) –Two step to enable automatic archiving Turn archive logging on –ALTER DATABASE ARCHIVELOG Set parameters in INIT.ORA –LOG_ARCHIVE_START = TRUE –LOG_ARCHIVE_DEST –LOG_ARCHIVE_FORMAT »%S »%s »%T »%t

The Component of a Database (20/21) Redo log files (8/9) –Multiple archive log destinations INIT.ORA parameters –LOG_ARCHIVE_DUPLEX_DEST –LOG_ARCHIVE_MIN_SUCCEED_DEST

The Component of a Database (21/21) Redo log files (9/9)

The Components of an Instance (1/14) An Oracle instance can be defined as –An area of shared memory System global area (SGA) –Change the SGA size »Prior to Oracle9i, stop and restart the instance. »With Oracle9i, while the Oracle instance is still running. –A collection of background processes

The Components of an Instance (2/14) Some physical files that can be considered as part of the instance –The instance initialization (INIT.ORA) file Many parameters may be altered dynamically. Oracle9i –SPFILE –The instance configuration (CONFIG.ORA) file Optional –The password file Optional

The Components of an Instance (3/14)

The Components of an Instance (4/14) Memory structure for an instance (1/4) –Database buffer cache Caches blocks of data retrieved from the database. “Least recently used” (LRU) algorithm. Waiting to perform I/O until absolutely necessary. Multiple buffer pools –DEFAULT »All objects use this cache unless otherwise indicated. –KEEP »For frequently used objects. –RECYCLE »For objects that are less likely to access again.

The Components of an Instance (5/14) Memory structure for an instance (2/4) –Shared pool Caches various constructs that can be shared among users. –SQL statements –Stored procedures –Data dictionary

The Components of an Instance (6/14) Memory structure for an instance (3/4) –Redo log buffer Caches redo information until it’s written to the physical redo log files.

The Components of an Instance (7/14) Memory structure for an instance (4/4) –Large pool Introduced in Oracle8 To reduce the demand on the shared pool for certain large memory allocation. –Buffering I/O for various server process –Storing session memory for the Multi-Threaded Server –Using the XA protocol for distributed transactions

The Components of an Instance (8/14) Background processes for an instance (1/7) –Database Writer (DBWR) Writes database blocks from the database buffer cache in the SGA to the datafiles on disk. Up to 10 DBWR processes, from DBW0 to DBW9. Two main reasons for writing blocks out of the cache: –To perform a check point. –To free space in the cache.

The Components of an Instance (9/14) Background processes for an instance (2/7) –Log Writer (LGWR) Writes the redo information from the log buffer in the SGA to all copies of the current redo log file on disk. When a transaction is committed, Oracle makes the redo information permanent by invoking the Log Writer to write it to disk.

The Components of an Instance (10/14) Background processes for an instance (3/7) –System Monitor (SMON) Maintains overall health and safety for an Oracle instance. –Performs crash recovery after an instance failure. –Coordinates and performs recovery for a failed instance when more than one instance accessing the same database. –Cleans up adjacent pieces of free space in the datafiles by merging them into one piece. –Gets rid of space used for sorting rows when the space is no longer needed.

The Components of an Instance (11/14) Background processes for an instance (4/7) –Process Monitor (PMON) Watches over the user processes that access the database. Cleaning up any of the resources and releasing any locks if a user process terminates abnormally.

The Components of an Instance (12/14) Background processes for an instance (5/7) –Archiver (ARCH) Reads the redo log files once Oracle has filled them and writes a copy of the used redo log files to the specified archive log destination(s). Up to 10 archiver process (ARCn).

The Components of an Instance (13/14) Background processes for an instance (6/7) –Checkpoint (CKPT) Performs checkpoints. Updates the control file and database file headers to update the checkpoint data when the checkpoint is complete.

The Components of an Instance (14/14) Background processes for an instance (7/7) –Recover (RECO) Automatically cleans up failed or suspended distributed transactions.

The Data Dictionary