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Basic Oracle Architecture

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					Basic Oracle Architecture
Presented to HIOUG
21-Jun-2005 By Byron Bush

Workshop 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”
Technically an Oracle Database is broken into two high-level components
1. INSTANCE – Non-persistent, memorybased processes and structures 2. DATABASE – Persistent, disk-based data and control files

So Why Differentiate?
Most Oracle installations consist of only a single “Instance” and single “Database”…
Instance

Database

Oracle RAC is Different
Oracle “Real Application Clusters” allow multiple “Instances” to interact with a single “Database” to provide high availability…
Instance 1 Instance 2 Instance 3

Database

“Instance” 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”
Instance SGA Shared Pool Library Cache Data Dictionary Cache Database Buffer Cache Redo Log Buffer Memory Structures Java Pool Large Pool

PMON

SMON

DBWR

LGWR

CKPT

Others

BackGround Processes

“Database” 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”
Oracle Database
Data Files
Parameter File Password File

Control Files

Redo Log Files
Archived Log Files

Picture of an “Instance”
Instance SGA Shared Pool Library Cache Data Dictionary Cache Database Buffer Cache Redo Log Buffer Memory Structures Java Pool Large Pool

PMON

SMON

DBWR

LGWR

CKPT

Others

BackGround Processes

Memory 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 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 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!

Picture of an “Instance”
Instance SGA Shared Pool Library Cache Data Dictionary Cache Database Buffer Cache Redo Log Buffer Memory Structures Java Pool Large Pool

PMON

SMON

DBWR

LGWR

CKPT

Others

BackGround Processes

Shared 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 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

Picture of an “Instance”
Instance SGA Shared Pool Library Cache Data Dictionary Cache Database Buffer Cache Redo Log Buffer Memory Structures Java Pool Large Pool

PMON

SMON

DBWR

LGWR

CKPT

Others

BackGround Processes

Data 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 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

Picture of an “Instance”
Instance SGA Shared Pool Library Cache Data Dictionary Cache Database Buffer Cache Redo Log Buffer Memory Structures Java Pool Large Pool

PMON

SMON

DBWR

LGWR

CKPT

Others

BackGround Processes

Redo 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 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 Pool
• Services parsing requirements for Java commands • Required if installing and using Java • Sized by JAVA_POOL_SIZE parameter

Oracle “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 Process
• A program that requests interaction with the Oracle server • Must first establish a connection • Does not interact directly with the Oracle server
User Process Server Process

Connection Established

Server Process
• A program that directly interacts with the Oracle server • Fulfills calls generated and returns results • Can be dedicated or shared server

Picture of an “Instance”
Instance SGA Shared Pool Library Cache Data Dictionary Cache Database Buffer Cache Redo Log Buffer Memory Structures Java Pool Large Pool

PMON

SMON

DBWR

LGWR

CKPT

Others

BackGround Processes

Background Processes
• Maintains and enforces relationships between physical and memory structures
– Mandatory background processes:
DBWn LGWR ARCn CJQ0 Dnnn LCKn PMON SMON LMDn LMON LMS Pnnn CKPT

– Optional background processes:
QMNn RECO Snnn

Database Writer (DBWn)
Instance Shared Pool Shared Pool Library Cache SGA Database Buffer Cache Java Pool Redo Log Buffer Large Pool

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

Data Dictionary Cache
PMON SMON

DBWR

LGWR

CKPT

Others

Oracle Database Data Files Control Files Redo Log Files

Log Writer (LGWR)
Instance Shared Pool Library Cache SGA Database Buffer Cache Java Pool Redo Log Buffer Large Pool

LGWR writes:
• • • • • At commit When one-third full When there is 1 MB of redo Every three seconds Before DBWn writes

Data Dictionary Cache
PMON SMON

DBWR

LGWR

CKPT

Others

Oracle Database Data Files Control Files Redo Log Files

System Monitor (SMON)
Instance Shared Pool Shared Pool Library Cache SGA Database Buffer Cache Java Pool Redo Log Buffer Large Pool

Responsibilities:
• Instance recovery
– Rolls forward changes in online redo log files – Opens database for user access – Rolls back uncommitted transactions

Data Dictionary Cache
PMON SMON

DBWR

LGWR

CKPT

Others

Oracle Database Data Files Control Files Redo Log Files

• •

Coalesces free space Deallocates temporary segments

Process Monitor (PMON)
Instance Shared Pool Shared Pool Library Cache SGA Database Buffer Cache Java Pool Redo Log Buffer Large Pool

Cleans up after failed processes by:
• • •
Others

Data Dictionary Cache
PMON SMON

DBWR

LGWR

CKPT

•

Rolling back the transaction Releasing locks Releasing other resources Restarting dead dispactchers

Oracle Database Data Files Control Files Redo Log Files

Checkpoint (CKPT)
Instance Shared Pool Shared Pool Library Cache SGA Database Buffer Cache Java Pool Redo Log Buffer Large Pool

Responsible for:
• • Signaling DBWn at checkpoints Updating datafile headers with checkpoint information Updating control files with checkpoint information

Data Dictionary Cache
PMON SMON

•
DBWR LGWR CKPT Others

Oracle Database Data Files Control Files Redo Log Files

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 Database Data Files Control Files Redo Log Files ARCn Archived Logs

Oracle 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

Tablespaces
• 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 Files
• Can belong to only one tablespace and one database • Are a repository for schema object data
Database

Tablespace
Data File Data File

2 Types of Tablespace
1. Tablespaces required by Oracle for normal operations 2. Tablespaces that contain data and indexes that support your applications

Required 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)
• • • • Insert Update Delete Select What happens when someone connects to the database and issues one of these statements?

Connect to Oracle – What Happens
User Process Connect User/Pwd Server Process

• 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 * From MyTab Order by 1;

• 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” – Parsing the Statement
Select * From MyTab Order by 1;

• 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 Statement
Select * From MyTab Order by 1;

• 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?
Insert (a,b,c) Into MyTab;

• 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

“I/U/D” – Executing the Statement
Insert (a,b,c) Into MyTab;

• 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

“I/U/D” – Executing the Statement
Insert (a,b,c) Into MyTab; Commit; or Rollback;

• 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

Archivelog Mode
• 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
Oracle Database Data Files Control Files Redo Log Files ARCn Archived Logs

Summary
• 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.


				
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Description: Oracle Architecture