Basic Oracle Architecture

Basic Oracle ArchitectureBasic Oracle Architecture

Presented to HIOUG Presented to HIOUG 21-Jun-200521-Jun-2005

By Byron BushBy Byron Bush

Workshop ObjectivesWorkshop Objectives

• Identify high-level architectural components of an Oracle Database

• Describe the function of each of these components

• Identify Oracle Database internal data structures

• Define the mechanisms used when processing Insert/Update/Delete statements

Most people call it a “Database”Most people call it a “Database”

Technically an Oracle Database is broken into two high-level components

1. INSTANCE – Non-persistent, memory-based processes and structures

2. DATABASE – Persistent, disk-based data and control files

So Why Differentiate?So Why Differentiate?

Most Oracle installations consist of only a single “Instance” and single “Database”…

Instance

Database

Oracle RAC is DifferentOracle RAC is Different

Oracle “Real Application Clusters” allow multiple “Instances” to interact with a single “Database” to provide high availability…

Instance 1

Database

Instance 3Instance 2

““Instance” a Little CloserInstance” a Little Closer

An Oracle Instance…– Is a means to access an Oracle Database– Always opens one and only one Database– Consists of memory and background process

structures

Picture of an “Instance”Picture of an “Instance”Instance

LibraryCache

DataDictionary

Cache

Shared PoolSGA

Database BufferCache

Redo LogBuffer

Java Pool Large Pool

PMON SMON DBWR LGWR CKPT Others

MemoryStructures

Back-GroundProcesses

““Database” a Little CloserDatabase” a Little Closer

An Oracle Database…– Is a collection of data that is treated as a unit– Consists of three file types

Picture of a “Database”Picture of a “Database”

ArchivedLog Files

ParameterFile

PasswordFile

Oracle Database

Data Files Control Files Redo Log Files


LibraryCache

DataDictionary

Cache

Shared PoolSGA


Redo LogBuffer



MemoryStructures


Memory StructureMemory Structure

Oracle’s memory structure consists of two memory areas known as:

1. System Global Area (SGA): Allocated at instance start up, and is a fundamental component of an Oracle Instance

2. Program Global Area (PGA): Allocated when the server process is started

System Global AreaSystem Global Area

• The SGA consists of several memory structures:– Shared Pool– Database Buffer Cache– Redo Log Buffer– Other structures

• There are two additional memory structures that can be configured within the SGA:– Large Pool– Java Pool

System Global AreaSystem Global Area

• The size of the SGA is determined by the parameters that set the sizes of the various pools; these parameters are dynamic

• The SGA_MAX_SIZE parameter sets the maximum size of the SGA (so you can limit it) and is not a dynamic parameter

Thanks Ned!


LibraryCache

DataDictionary

Cache

Shared PoolSGA


Redo LogBuffer



MemoryStructures


Shared PoolShared Pool

• Used to store:– Most recently executed SQL statements– Most recently used data definitions

• It consists of two key performance-related memory structures:– Library Cache– Data Dictionary Cache

• Sized by the parameter SHARED_POOL_SIZE

Library CacheLibrary Cache

• Stores information about the most recently used SQL and PL/SQL statements

• Enables the sharing of commonly used statements

• Is managed by a least recently used (LRU) algorithm

• Consists of two structures– Shared SQL area– Shared PL/SQL area

• Size is determined by the Shared Pool sizing


LibraryCache

DataDictionary

Cache

Shared PoolSGA


Redo LogBuffer



MemoryStructures


Data Dictionary CacheData Dictionary Cache

• A collection of the most recently used definitions in the database

• Includes information about database files, tables, indexes, columns, users, privileges, and other database objects

• During the parse phase, the server process looks at the data dictionary for information to resolve object names and validate access

Database Buffer CacheDatabase Buffer Cache

• Stores copies of data blocks that have been retrieved from the data files

• Enables great performance gains when you obtain and update data

• Managed through an LRU algorithm

• DB_BLOCK_SIZE determines primary block size


LibraryCache

DataDictionary

Cache

Shared PoolSGA


Redo LogBuffer



MemoryStructures


Redo Log BufferRedo Log Buffer

• Records all changes made to the database data blocks

• Primary purpose is recovery

• Changes recorded within are called redo entries

• Redo entries contain information to reconstruct or redo changes

• Size defined by LOG_BUFFER

Large PoolLarge Pool

• An optional area of memory in the SGA• Relieves the burden place on the Shared Pool• Used for:

– Session memory for the Shared Server– I/O server processes– Backup and restore operations for RMAN

• Does not use an LRU list• Sized by LARGE_POOL_SIZE• Can be dynamically resized

Java PoolJava Pool

• Services parsing requirements for Java commands

• Required if installing and using Java

• Sized by JAVA_POOL_SIZE parameter

Oracle “Process” StructureOracle “Process” Structure

• Oracle takes advantage of various types of Processes:– User Process: Started at the time a database

user requests connection to the Oracle Server– Server Process: Connects to the Oracle

instance and is started when a user establishes a session

– Background Processes: Started when an Oracle instance is started

User ProcessUser Process

• A program that requests interaction with the Oracle server

• Must first establish a connection• Does not interact directly with the Oracle server

ServerProcess

UserProcess

ConnectionEstablished

Server ProcessServer Process

• A program that directly interacts with the Oracle server

• Fulfills calls generated and returns results

• Can be dedicated or shared server


LibraryCache

DataDictionary

Cache

Shared PoolSGA


Redo LogBuffer



MemoryStructures


Background ProcessesBackground Processes

• Maintains and enforces relationships between physical and memory structures– Mandatory background processes:

DBWn PMON CKPTLGWR SMON

– Optional background processes:ARCn LMDn QMNnCJQ0 LMON RECODnnn LMS SnnnLCKn Pnnn

Database Writer (DBWn)Database Writer (DBWn)

DBWn writes when:• Checkpoint occurs

• Dirty buffers reach threshold

• There are no free buffers

• Timeout occurs

• Tablespace OFFLINE

• Tablespace READ ONLY

• Table DROP or TRUNCATE

• Tablespace BEGIN BACKUP

Oracle DatabaseData Files Control Files Redo Log Files

Instance

Shared Pool

LibraryCache

DataDictionary

Cache

SGA


Redo LogBuffer



Shared Pool

Log Writer (LGWR)Log Writer (LGWR)

LGWR writes:• At commit

• When one-third full

• When there is 1 MB of redo

• Every three seconds

• Before DBWn writes

Instance

Shared Pool

LibraryCache

DataDictionary

Cache

SGA


Redo LogBuffer




System Monitor (SMON)System Monitor (SMON)

Responsibilities:• Instance recovery

– Rolls forward changes in online redo log files

– Opens database for user access

– Rolls back uncommitted transactions

• Coalesces free space

• Deallocates temporary segments


Instance

Shared Pool

LibraryCache

DataDictionary

Cache

SGA


Redo LogBuffer



Shared Pool

Process Monitor (PMON)Process Monitor (PMON)

Cleans up after failed processes by:

• Rolling back the transaction

• Releasing locks

• Releasing other resources

• Restarting dead dispactchers



Instance

Shared Pool

LibraryCache

DataDictionary

Cache

SGA

Redo LogBuffer



Shared Pool

Checkpoint (CKPT)Checkpoint (CKPT)

Responsible for:• Signaling DBWn at

checkpoints

• Updating datafile headers with checkpoint information

• Updating control files with checkpoint information

Instance

Shared Pool

LibraryCache

DataDictionary

Cache

SGA


Redo LogBuffer




Shared Pool

Archiver (ARCn)Archiver (ARCn)• Optional background process• Automatically archives online redo log files when

ARCHIVELOG mode is set• Preserves the record of all changes made to the

database

Oracle DatabaseData Files Control Files Redo Log Files Archived Logs

ARCn

Oracle Data StorageOracle Data Storage

Oracle keeps all system and user data in two basic storage containers:

1. Tablespace… a logical entity known only to Oracle

2. Data Files… physical files that may (typically) be seen from the operating system

TablespacesTablespaces

• Can belong to only one database at a time

• Consist of one or more data files

• Are further divided into logical units of store

Data FilesData Files

• Can belong to only one tablespace and one database

• Are a repository for schema object data

DataFile

DataFile

Database

Tablespace

2 Types of Tablespace2 Types of Tablespace

1. Tablespaces required by Oracle for normal operations

2. Tablespaces that contain data and indexes that support your applications

Required TablespacesRequired Tablespaces

• System – Contains the Oracle Data Dictionary

• Sysaux - New to 10g; supports historic monitoring / tuning

• Temporary – Used for disk-based sorting of data (select...from…order by)

• Undo – Used for transaction consistency during Insert / Update / Delete statements

SQL Statements (IUDS)SQL Statements (IUDS)

• Insert

• Update

• Delete

• Select

What happens when someone connects to the database and issues one of these statements?

Connect to Oracle – What HappensConnect to Oracle – What Happens

ServerProcess

UserProcess

Connect User/Pwd

• Are “User” definition and privileges in SGA “Dictionary Cache”?– If not, try to fetch from the “System”

tablespace– If there, validate Pwd and privileges to login– Allow connection if all is right; otherwise

decline connection

““Select” – What Happens First?Select” – What Happens First?

• Is this statement in the “Library Cache”?– If statement cached then it has been recently

been “parsed” and “executed”– If not cached then “parse” the statement– If cached then skip parsing stage and execute

statement

Select *From MyTabOrder by 1;

““Select” – Parsing the StatementSelect” – Parsing the Statement

• Does “MyTab” exist?

• Does user have Privs to select from “MyTab”

• What columns are in “MyTab”

• What is the first column in “MyTab”


““Select” – Executing the StatementSelect” – Executing the Statement

• Is “MyTab” data in the SGA “Buffer Cache”?– If not, fetch data into Buffer Cache

• Sort data in “MyTab” by the first column– If can sort records in memory then do so– If cannot sort in memory then use “Temporary”

tablespace as disk-based staging area

• Return records to client process


““I/U/D” – What Happens First?I/U/D” – What Happens First?

• Is this statement in the “Library Cache”?– If statement cached then it has been recently

been “parsed” and “executed”– If not cached then “parse” the statement– If cached then skip parsing stage and execute

statement

Insert (a,b,c)Into MyTab;

““I/U/D” – Executing the StatementI/U/D” – Executing the Statement

• Validate values to be inserted• Execute the statement

– Keep DB version of the record in “Undo” tablespace until Commit or Rollback

– Record changes in SGA “Redo Log Buffer”– Change records in SGA “Buffer Cache”– DBWn writes changed records to data file(s)

as part of buffer cache management

Insert (a,b,c)Into MyTab;

““I/U/D” – Executing the StatementI/U/D” – Executing the Statement

• If “Commit” then…– LGWn writes “Redo Log Buffer” entries to

“Redo Logs”– Undo entries are invalidated

• If “Rollback” then…– Migrate DB version of record in Undo back to

tablespace/data file

Insert (a,b,c)Into MyTab;Commit;orRollback;

Archivelog ModeArchivelog Mode

Oracle DatabaseData Files Control Files Redo Log Files Archived Logs

ARCn

• If Instance in “Archivelog” mode Redo Log Files are “Archived” to Archive Logs; which may be used to recover your database in the case of disaster

SummarySummary• What we called a “Database” is really an

“Instance” and a “Database.”• An Oracle “Instance” is not persistent. It consists

of the System Global Area (SGA) and a collection of processes.

• An Oracle “Database” is persistent. It consists of a collection of “Required” and “User” tablespaces (with corresponding data files) along with other supporting files.

• Nearly every component of the “Instance” and “Database” are mobilized to execute SQL statements.

Documents

Basic Oracle Architecture