Oracle architectural components
一、.Overview of Primary Components
The Oracle architecture includes a number of primary components,as listed:
a.Oracle server
includes several files，processes,and memory structures.The Oracle server consists of an Oracle instance and an Oracle database.
b.Oracle instance
includes background processes,memory structures.such as SGA includes Shared pool(library cache,data dictionary cache),database buffer cache,redo log buffer cache. Background includes PMON,SMON,DRWR,LGWR,CKPT,OTHERS and so on.
is a means to access an Oracle database
always opens one and only one database
consists of memory and process structures
Memory Structures——SGA:Shared pool(library cache,data dictionary chache),database buffer cache,redo log buffer cache
Background structures:PMON,SMON,DBWR,LGWR,CKPT,Others
establishing a connection and creating a session
c.Oracle Database
includes operating system files,also known as database files.
is a collection of data that is treated as a unit
consists of three file types:
Data files containing the actual data in the database
redo logs containing a record of changes made to the database to enable recovery of the data in case of failures
control files containing information necessary to maintain and verify database integrity
d.Other keys files
configure the instance,anthenticate privileged users, and recover the database in the event of a disk failure.
the parameter file defines the characteristics of an Oracle instance.
the password file authenticates users privileged to start up and shut down an Oracle instance
archived redo log files are offline copies of the redo log files that may be necessary to recover from media failures.
e.User and server process
execute a SQL statement.
f.Other process
Many other processes,other options within Oracle,such as Advanced Queuing,Real Application Clusters,Shared Server,Advanced Replication and so on.
二、physical structure
The physical structure of an Oracle database is determined by the operating system files that provide the actual physical storage for database information
control files
data files
redo log files
三、Memory Structure
System Global Area(SGA)
Oracle's memory structure consists of two memory areas known as :
1.System Global Area(SGA):Allocated at instance startup,and is a fundamental component of an Oracle instance
shared pool
redo log buffer
database buffer cache
other structures(e.g.lock and latch management,statistical data)
There are two optional memory structures that can be configured within the SGA
Large pool
Java pool
2.Program Global Area(PGA):Allocated when the server process is started
example:
SQL> show sga;
Total System Global Area 118255568 bytes
Fixed Size 282576 bytes
Variable Size 83886080 bytes
Database Buffers 33554432 bytes
Redo Buffers 532480 bytes
SQL> 532480 bytes
四、Share Pools
The shared pool is used to store the most recently executed SQL statements and the most recently used data definitions.
1.It consists of two key performance-related memory structures:
Library cache
Data dictionary cache
2.Size by the parameter: SHARED_POOL_SIZE.
alter system set shared_pool_size=
the shared pool environment contains both fixed and variable structures.
the library cache stores information about the most recently used SQL and PL/SQL statements.
the library cache:
Enables the sharing of commonly used (LRU).
Is managed by a least recently used (LRU) algorithm
Consists of two structures:
Shared SQL area
Shared PL/SQL area;
Has its size determined by the shared pool sizing
Data Dictionary Cache
The data dictionary cache is a collection of the most recently used definitions in the database.
it 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.
Caching the data dictionary information into memory improves response time on queries.
Size is determined by the shared pool sizeing
五、Database buffer cache
The database buffer cache stores copies of data blocks that have been retrieved from the data files
it enables great performance gains when you obtain an update data.
It is managed through a least recently used (LRU) algorithm.
DB_BLOCK_SIZE determines the primary block size.
Consists of independent sub-caches:
----DB_CACHE_SIZE
----DB_KEEP_CACHE_SIZE
----DB_RECYCLE_CACHE_SIZE
Database buffer cache can be dynamically resized to grow or shrink using ALTER SYSTEM.
ALTER SYSTEM SET DB_CACHE_SIZE =
DB_CACHE_ADVICE can be set gather statistics for predicting different cache size behavior.
六、Redo Log Buffer Cache
The redo log buffer cache records all changes made to the database data blocks.
Its primary purpose is recovery.
Changes recorded within are called redo entries.
Redo entries contain information to reconstruct or redo changes.
Size is defined by LOG_BUFFER.
INITIALIZATION PARAMETER : LOG_BUFFER
七、Large Pool
The large pool is an optional area of memory in the SGA configured only in a shared server environment.
It relieves the burden placed on the shared pool.
This configured memory area is used for session memory (UGA),I/O slaves, and backup and restore operations.
Unlike the shared pool,the large pool does not use an LRU list.
Sized by LARGE_POOL_SIZE.
八、Java Pool (optional)
The Java pool services the parsing requirements for java commands.
Required if installing and using java
It is stored much the same way as PL/SQL in database tables
It is sized by the JAVA_POOL_SIZE parameter.
In Oracle9i,the default size of the java Pool is
Program Global Area (PGA)
九、Program global area
The SGA is memory reserved for each user process that connects to an Oracle database.
The PGA includes these components：
Sort area----Used for any sorts that may be required to process the SQL statement
Session information----Includes user privileges and performance statistics for the session
Cursor state----Indicates the stage in the processing of the SQL statements that are currently used by the session.
Stack space----Contains other session variables
Some of these structures are stored in the SGA when using shared server configuration .If using a shared server configuration .It is possible for multiple user processes to share server processes. If a large pool is created , the structures are stored in the large pool ; otherwise ,they are stored in the shared pool.
十、Process Structure
An Oracle process is a program that depending on its type can request information,execute a series of steps , or perform a specific task.
Oralce takes advantage of various types of processes:
1. User process:Started at the time a database user requests connection to the Oracle server
2. Server process:Connects to the Oracle Instannce and is started when a user establishes a session.
3. Background process:Available when an Oracle instance is started.
User Process
A user process is a program that requests interaction with the Oralce server.
It must first establish a connection.(UPI)
It does not interact directly with the Oracle server.
Server Process
A Server process is a program that directly interacts with the Oracle server
It fulfills calls generated and returns results.
Can be dedicated or shared server.
Background Processes
The relationship between the physical and memory structures is maintained and enforced by Oracle’s background processes.
Mandatory background processes
DBWn PMON CKPT
LGWR SMON RECO
Optional background processes
ARCn LMON Snnn
QMNn LMDn
CJQ0 Pnnn
LCKn Dnnn
Database writer(DBWn)
DBWn writes when
Checkpoint
Dirty buffers threshold reached
No free buffers
Timeout
RAC ping request
Tablespace read only
Table Drop or Truncate
Talbespace begin backup
Log Writer(LGWR)
LGWR writes:
At commit
When one-third full
When there is 1MB of redo
Every 3 seconds
Before DBWn writes
System Monitor (SMON)
Responsibilities:
Instance recovery:
--Rolls forward changes in the redo logs
--Opens the database for user access
--Rolls back uncommitted transactions
Coalesces free space ever 3 sec
Deallocates temporary segments
Process monitor (PMON)
Cleans up after failed processes by :
Rollling back the transaction
Releasing locks
Releasing other resources
Restarts dead dispatchers
Checkpoint (CKPT)
Responsible for :
Signaling DBWn at checkpoints
Updating datafile headers with checkpoint information
Updating control files with checkpoint information
Archiver (ARCn)
Optional background process
Automatically archives online redo logs when ARCHIVELOG mode is set
Preserves the record of all changes made to the database
Logical structure
The logical structure of the oracle architecture dictates how the physical space of a database is to be used.
A hierarchy exists in this structure that consists of tablespaces,segments,extents,and blocks.
Logical structure
A logical structure hierarchy exists as follows:
An oracle database is a group of tablespace.
A tablespace may consist of one more segments.
A segment is made up of extents.
An extent is made up of logical blocks.
A block is the smallest unit for read and write operations.
Processing a sql statement
Connect to an instance using
The user process
The server process
The oracle server components that are used depend on the type of SQL statement:
--Queries return rows
--DML statements log changes.
--Commit ensures transaction recovery
Some oracle server components do not participate in SQL statement processing.