Oracle architecture

Module 1 – Oracle Architecture
Objectives
These notes introduce the Oracle server architecture. The architecture includes physical components,
memory components,processes,and logical structures.

Primary Architecture Components

The figure shown above details the Oracle architecture.
Oracle server: An Oracle server includes an Oracle Instance and an Oracle database.
 An Oracle database includes several differenttypes of files: datafiles, control files,redo log files
and archive redo log files. The Oracle server also accessesparameter files and password files.
 This set of files has several purposes.
o One is to enable system users to process SQL statements.
o Another is to improve system performance.
o Still another is to ensure the database can be recovered if there is a software/hardware
failure.
 The database server must manage large amounts of data in a multi-user environment.
 The server must manage concurrent access to the same data.
 The server must deliver high performance. This generally means fast response times.
Oracle instance: An Oracle Instance consists of two differentsets of components:
 The first componentset is the set of backgroundprocesses (PMON, SMON, RECO,DBW0,
LGWR,CKPT,D000 and others).
o These will be covered later in detail – each background process is a computerprogram.
o These processes perform input/output and monitor other Oracle processes to provide good
performance and database reliability.
 The second componentset includes the memorystructures that comprise the Oracle instance.
o When an instance starts up, a memory structure called the System Global Area (SGA) is
allocated.
o At this point the background processesalso start.
 An Oracle Instance provides access to one and only one Oracle database.

Oracle database: An Oracle database consists of files.
 Sometimes these are referred to as operating system files,but they are actually database
files that store the database information that a firm or organization needs in order to operate.
 The redo log files are used to recoverthe database in the event of application program failures,
instance failures and other minor failures.
 The archived redolog files are used to recoverthe database if a disk fails.
 Other files not shown in the figure include:
o The required parameter file that is used to specifyparameters for configuring an Oracle
instance when it starts up.
o The optional password file authenticates specialusers of the database – these are
termed privileged users and include database administrators.
o Alert and TraceLog Files – these files store information about errors and actions taken that
affectthe configuration of the database.
User and server processes: The processesshown in the figure are
called user and serverprocesses. These processes are used to manage the execution of SQL
statements.
 A SharedServer Process canshare memoryand variable processing formultiple user
processes.
 A DedicatedServer Process manages memoryand variables for a single user process.
This figure from the Oracle DatabaseAdministration Guide provides another way of viewing the SGA.

Connecting to an Oracle Instance – Creating a Session
System users can connectto an Oracle database through SQLPlus or through an application program
like the Internet DeveloperSuite (the program becomes the system user). This connectionenables
users to execute SQL statements.
The act of connecting creates a communication pathway between a user process and an Oracle
Server. As is shown in the figure above, the User Process communicateswith the Oracle Server
through a Server Process. The User Process executes on the client computer. The Server Process
executes on the server computer,and actually executes SQL statements submitted by the system user.

The figure shows a one-to-one correspondence betweenthe User and Server Processes. This is called
a Dedicated Server connection. An alternative configuration is to use a Shared Server where more
than one User Processshares a Server Process.
Sessions: Whena user connects to an Oracle server, this is termed a session. The User Global
Area is sessionmemory and these memorystructures are described later in this document. The
sessionstarts when the Oracle server validates the user for connection. The sessionends when the
user logs out (disconnects)or if the connectionterminates abnormally (network failure or client computer
failure).
A user can typically have more than one concurrent session,e.g., the user may connectusing SQLPlus
and also connectusing Internet DeveloperSuite tools at the same time. The limit of concurrent session
connections is controlled by the DBA.
If a system users attempts to connect and the Oracle Server is not running, the system user receives
the Oracle Not Available error message.
Physical Structure – Database Files
As was noted above, an Oracle database consists of physical files. The database itself has:
 Datafiles – these contain the organization's actual data.
 Redo log files – these contain a chronological record of changes made to the database, and
enable recovery when failures occur.
 Controlfiles – these are used to synchronize all database activities and are covered in more
detail in a later module.

Other key files as noted above include:
 Parameter file – there are two types of parameter files.
o The init.ora file (also called the PFILE)is a static parameter file. It contains parameters
that specifyhow the database instance is to start up. For example, some parameters will
specifyhow to allocate memory to the various parts of the system global area.
o The spfile.ora is a dynamic parameter file. It also stores parameters to specifyhow to
startup a database; however, its parameters can be modified while the database is running.
 Password file – specifies which *special* users are authenticated to startup/shut down an Oracle
Instance.

 Archived redo log files – these are copies of the redo log files and are necessaryforrecovery in an
online, transaction-processing environment in the event of a disk failure.
Memory Management and Memory Structures
Oracle Database Memory Management
Memory management - focus is to maintain optimal sizes for memorystructures.
 Memory is managed based on memory-related initialization parameters.
 These values are stored in the init.ora file for each database.
Three basic options for memory management are as follows:
 Automatic memory management:
o DBA specifies the target size for instance memory.
o The database instance automatically tunes to the target memory size.
o Database redistributes memory as needed betweenthe SGA and the instance PGA.
 Automatic sharedmemory management:
o This management mode is partially automated.
o DBA specifies the target size for the SGA.
o DBA can optionally set an aggregate target size for the PGA or managing PGA work areas
individually.
 Manualmemory management:
o Instead of setting the total memory size, the DBA sets many initialization parameters to
manage components ofthe SGA and instance PGA individually.

If you create a database with Database Configuration Assistant (DBCA) and choose the basic
installation option, then automatic memory management is the default.
The memory structures include three areas of memory:
 System Global Area (SGA) – this is allocated when an Oracle Instance starts up.
 Program Global Area (PGA) – this is allocated when a Server Process starts up.
 User Global Area (UGA) – this is allocated when a user connects to create a session.
System Global Area
The SGA is a read/write memory area that stores information shared by all database processes and by
all users of the database (sometimesit is called theShared GlobalArea).
o This information includes both organizational data and control information used by the Oracle
Server.
o The SGA is allocated in memoryand virtual memory.
o The size of the SGA can be established by a DBA by assigning a value to the
parameter SGA_MAX_SIZE in the parameter file—this is an optional parameter.
The SGA is allocated when an Oracle instance (database) is started up based on values specifiedin the
initialization parameter file (either PFILE or SPFILE).
The SGA has the following mandatory memorystructures:
 Database BufferCache
 Redo Log Buffer
 Java Pool
 Streams Pool
 Shared Pool – includes two components:

o Library Cache
o Data Dictionary Cache
 Other structures (for example, lock and latch management, statistical data)
Additional optional memorystructures in the SGA include:
 Large Pool
The SHOW SGA SQL command will show you the SGA memory allocations.
 This is a recent clip of the SGA for the DBORCL database at SIUE.
 In order to execute SHOW SGA you must be connected with the specialprivilege SYSDBA(which
is only available to user accounts that are members of the DBA Linux group).
SQL> connect / as sysdba
Connected.
SQL> show sga
Total System Global Area 1610612736 bytes
Fixed Size 2084296 bytes
Variable Size 1006633528 bytes
Database Buffers 587202560 bytes
Redo Buffers 14692352 bytes
Early versions of Oracle used a Static SGA. This meant that if modifications to memorymanagement
were required, the database had to be shutdown, modifications were made to the init.ora parameter file,
and then the database had to be restarted.

Oracle 11g uses a Dynamic SGA. Memory configurations for the system global area can be made
without shutting down the database instance. The DBA can resize the Database BufferCache and
Shared Pool dynamically.
Several initialization parameters are set that affectthe amount of random access memorydedicated to
the SGA of an Oracle Instance. These are:
 SGA_MAX_SIZE: This optional parameter is used to set a limit on the amount of virtual
memory allocated to the SGA – a typical setting might be 1 GB; however, if the value
for SGA_MAX_SIZEin the initialization parameter file or server parameter file is less than the sum
the memory allocated for all components,either explicitly in the parameter file or by default, at the
time the instance is initialized, then the database ignores the setting for SGA_MAX_SIZE. For
optimal performance,the entire SGA should fit in real memory to eliminate paging to/from disk by
the operating system.
 DB_CACHE_SIZE: This optional parameter is used to tune the amount memoryallocated to the
Database BufferCache in standard database blocks. Block sizes vary among operating
systems. The DBORCL database uses 8 KB blocks. The total blocks in the cache defaults to 48
MB on LINUX/UNIX and 52 MB on Windows operating systems.
 LOG_BUFFER: This optional parameter specifies the number of bytes allocated for the Redo
Log Buffer.
 SHARED_POOL_SIZE: This optional parameter specifiesthe number of bytes of memory
allocated to shared SQL and PL/SQL. The default is 16 MB. If the operating system is based on
a 64 bit configuration, then the default size is 64 MB.
 LARGE_POOL_SIZE: This is an optional memory object – the size of the Large Pool defaults to
zero. If the init.ora parameterPARALLEL_AUTOMATIC_TUNING is set to TRUE, then the default
size is automatically calculated.
 JAVA_POOL_SIZE: This is another optional memory object. The default is 24 MB of memory.

The size of the SGA cannot exceed the parameter SGA_MAX_SIZE minus the combination of the size
of the additional
parameters, DB_CACHE_SIZE,LOG_BUFFER,SHARED_POOL_SIZE,LARGE_POOL_SIZE,
and JAVA_POOL_SIZE.
Memory is allocated to the SGA as contiguous virtual memory in units termed granules. Granule size
depends on the estimated total size of the SGA, which as was noted above, depends onthe
SGA_MAX_SIZEparameter. Granules are sized as follows:
 If the SGA is less than 1 GB in total, each granule is 4 MB.
 If the SGA is greater than 1 GB in total, each granule is 16 MB.
Granules are assigned to the Database BufferCache, Shared Pool, Java Pool, and other memory
structures, and these memorycomponents can dynamically grow and shrink. Using contiguous memory
improves system performance. The actual number of granules assigned to one of these memory
components can be determined by querying the database view named V$BUFFER_POOL.
Granules are allocated when the Oracle server starts a database instance in order to provide memory
addressing space to meet the SGA_MAX_SIZE parameter. The minimum is 3 granules: one each for
the fixed SGA, Database BufferCache, and Shared Pool. In practice,you'll find the SGA is allocated
much more memorythan this. The SELECT statementshown below shows a current_size of 1,152
granules.
SELECT name, block_size, current_size, prev_size, prev_buffers
FROM v$buffer_pool;
NAME BLOCK_SIZE CURRENT_SIZE PREV_SIZE PREV_BUFFERS
-------------------- ---------- ------------ ---------- ------------
DEFAULT 8192 560 576 71244

For additional information on the dynamic SGA sizing, enroll in Oracle's Oracle11gDatabase
PerformanceTuningcourse.
Program Global Area (PGA)
A PGA is:
 a nonsharedmemoryregion that contains data and control information exclusively for use by an
Oracle process.
 A PGA is created by Oracle Database when an Oracle process is started.
 One PGA exists for each ServerProcessand each BackgroundProcess. It stores data and
control information for a single Server Process ora single Background Process.
 It is allocated when a processis created and the memory is scavenged by the operating system
when the process terminates. This is NOT a shared part of memory – one PGA to each process
only.
 The collectionof individual PGAs is the total instance PGA, or instance PGA.
 Database initialization parameters set the size of the instance PGA, not individual PGAs.
The ProgramGlobalArea is also termed the Process GlobalArea (PGA) and is a part of memory
allocated that is outside of the Oracle Instance.

The content of the PGA varies, but as shown in the figure above, generally includes the following:
 Private SQL Area: Stores information for a parsed SQL statement – stores bind variable values and
runtime memory allocations. A user sessionissuing SQL statements has a Private SQL Area that
may be associated with a Shared SQL Area if the same SQL statement is being executed by more
than one system user. This oftenhappens in OLTP environments where many users are executing
and using the same application program.
o Dedicated Serverenvironment – the Private SQL Area is located in the Program Global Area.
o Shared Serverenvironment – the Private SQL Area is located in the System Global Area.
 Session Memory: Memory that holds sessionvariables and other sessioninformation.
 SQL Work Areas: Memory allocated for sort, hash-join, bitmap merge,and bitmap create types of
operations.
o Oracle 9i and later versions enable automatic sizing of the SQL WorkAreas by setting
the WORKAREA_SIZE_POLICY = AUTO parameter (this is the default!)
and PGA_AGGREGATE_TARGET = n (where n is some amount of memory established by the
DBA). However, the DBA can let the Oracle DBMS determine the appropriate amount of
memory.

User Global Area
The User GlobalArea is sessionmemory.
A sessionthat loads a PL/SQL packageinto memoryhas the package state stored to the
UGA. The packagestate is the set of values stored in all the package variables at a specific time. The
state changes as program code the variables. By default, package variables are unique to and persist
for the life of the session.
The OLAP page pool is also stored in the UGA. This pool manages OLAP data pages,which are
equivalent to data blocks.The page pool is allocated at the start of an OLAP sessionand released at the
end of the session. An OLAP sessionopens automatically whenever a user queries a dimensional
objectsuch as a cube.
Note: Oracle OLAP is a multidimensional analytic engine embeddedin Oracle Database
11g. Oracle OLAP cubes deliver sophisticated calculations using simple SQL queries - producing
results with speed ofthought response times.

The UGA must be available to a database sessionforthe life of the session. For this reason, the UGA
cannot be stored in the PGA when using a shared server connection because the PGA is specificto a
single process. Therefore,the UGA is stored in the SGA when using shared server connections,
enabling any shared server process accessto it. When using a dedicated server connection, the UGA is
stored in the PGA.
Automatic Shared Memory Management
Prior to Oracle 10G,a DBA had to manually specifySGA Componentsizes through the initialization
parameters, such as SHARED_POOL_SIZE,DB_CACHE_SIZE,JAVA_POOL_SIZE,and
LARGE_POOL_SIZE parameters.
Automatic Shared Memory Management enables a DBA to specifythe total SGA memory available
through the SGA_TARGET initialization parameter. The Oracle Database automatically distributes this
memory among various subcomponents to ensure most effective memoryutilization.
The DBORCL database SGA_TARGET is set in the initDBORCL.orafile:
sga_target=1610612736
With automatic SGA memory management, the differentSGA components are flexibly sized to adapt to
the SGA available.
Setting a single parameter simplifies the administration task – the DBA only specifies the amount of SGA
memory available to an instance – the DBA can forgetabout the sizes of individual components.No out
of memoryerrors are generated unless the system has actually run out of memory. No manual tuning
effortis needed.

The SGA_TARGET initialization parameter reflects the total size of the SGA and includes memoryfor
the following components:
 Fixed SGA and other internal allocations needed by the Oracle Database instance
 The log buffer
 The shared pool
 The Java pool
 The buffercache
 The keep and recycle buffercaches (if specified)
 Nonstandard block size buffercaches (if specified)
 The Streams Pool
If SGA_TARGET is set to a value greater than SGA_MAX_SIZE at startup, then the SGA_MAX_SIZE
value is bumped up to accommodate SGA_TARGET.

When you set a value for SGA_TARGET,Oracle Database 11g automatically sizes the most commonly
configured components,including:
 The shared pool(for SQL and PL/SQL execution)
 The Java pool (for Java execution state)
 The large pool (for large allocations such as RMAN backup buffers)
 The buffercache
There are a few SGA components whose sizes are not automatically adjusted. The DBA must specify
the sizes of these components explicitly, if they are needed by an application. Such components are:
 Keep/Recyclebuffercaches (controlled
by DB_KEEP_CACHE_SIZE and DB_RECYCLE_CACHE_SIZE)
 Additional buffercaches for non-standard block sizes (controlled by DB_nK_CACHE_SIZE,n = {2,
4, 8, 16, 32})
 Streams Pool(controlled by the new parameter STREAMS_POOL_SIZE)
The granule size that is currently being used for the SGA for each componentcan be viewed in the
view V$SGAINFO.The size of each componentand the time and type of the last resize operation
performedon each componentcan be viewed in the view V$SGA_DYNAMIC_COMPONENTS.
SQL> select * from v$sgainfo;
More...
NAME BYTES RES
-------------------------------- ---------- ---
Fixed SGA Size 2084296 No
Redo Buffers 14692352 No
Buffer Cache Size 587202560 Yes
Shared Pool Size 956301312 Yes
Large Pool Size 16777216 Yes

Java Pool Size 33554432 Yes93
Streams Pool Size 0 Yes
Granule Size 16777216 No
Maximum SGA Size 1610612736 No
Startup overhead in Shared Pool 67108864 No
Free SGA Memory Available 0
11 rows selected.
Shared Pool

The Shared Pool is a memorystructure that is shared by all system users.
 It caches various types of program data. For example, the shared pool stores parsed SQL,
PL/SQL code,system parameters, and data dictionaryinformation.

 The shared poolis involved in almost every operation that occurs in the database. For example, if
a user executes a SQL statement, then Oracle Database accessesthe shared pool.
 It consists of both fixed and variable structures.
 The variable componentgrows and shrinks depending on the demands placed on memory size by
system users and application programs.
Memory can be allocated to the Shared Pool by the parameter SHARED_POOL_SIZE in the parameter
file. The default value of this parameter is 8MB on 32-bit platforms and 64MB on 64-bitplatforms.
Increasing the value of this parameter increases the amount of memory reserved forthe shared pool.
You can alter the size of the shared pooldynamically with the ALTERSYSTEM SET command. An
example command is shown in the figure below. You must keep in mind that the total memoryallocated
to the SGA is set by the SGA_TARGET parameter (and may also be limited by the SGA_MAX_SIZE if it
is set), and since the Shared Poolis part of the SGA, you cannot exceed the maximum size of the
SGA. It is recommended to let Oracle optimize the Shared Pool size.
The Shared Poolstores the mostrecently executed SQL statements and used data definitions. This is
because some system users and application programs will tend to execute the same SQL statements
often. Saving this information in memory can improve system performance.
The Shared Poolincludes several cache areas described below.
Library Cache
Memory is allocated to the Library Cache whenever an SQL statement is parsed or a program unit is
called. This enables storage of the mostrecently used SQL and PL/SQL statements.

If the Library Cache is too small, the Library Cache must purge statement definitions in order to have
space to load new SQL and PL/SQL statements. Actual management of this memory structure is
through a Least-Recently-Used (LRU) algorithm. This means that the SQL and PL/SQL statements
that are oldestand least recently used are purged when more storage space is needed.
The Library Cache is composed of two memorysubcomponents:
 Shared SQL: This stores/shares the execution plan and parse tree for SQL statements, as well
as PL/SQL statements such as functions, packages,and triggers. If a system user executes an
identical statement, then the statement does not have to be parsed again in order to execute the
statement.
 Private SQL Area: With a shared server, each sessionissuing a SQL statement has a private
SQL area in its PGA.
o Each user that submits the same statement has a private SQL area pointing to the same
shared SQL area.
o Many private SQL areas in separate PGAs can be associated with the same shared SQL
area.
o This figure depicts two differentclient processes issuing the same SQL statement – the
parsed solution is already in the Shared SQL Area.

The Data Dictionary Cache is a memorystructure that caches data dictionary information that has been
recently used.
 This cache is necessary because the data dictionary is accessedso often.
 Information accessed includes user account information, datafile names, table descriptions,user
privileges,and other information.
The database server manages the size of the Data Dictionary Cache internally and the size depends on
the size of the Shared Poolin which the Data Dictionary Cache resides. If the size is too small, then the
data dictionary tables that reside on disk must be queried oftenfor information and this will slow down
performance.
Server Result Cache
The Server Result Cache holds result sets and not data blocks.The server result cache contains
the SQL query result cache and PL/SQL function result cache, which share the same infrastructure.
SQL Query Result Cache
This cache stores the results of queries and query fragments.
 Using the cache results for future queries tends to improve performance.
 For example, suppose an application runs the same SELECT statementrepeatedly.If the results
are cached, then the database returns them immediately.
 In this way, the database avoids the expensive operation of rereading blocks and recomputing
results.
PL/SQL Function Result Cache
The PL/SQL Function Result Cache stores function result sets.

 Without caching, 1000 calls of a function at 1 second per call would take 1000 seconds.
 With caching, 1000 function calls with the same inputs could take 1 second total.
 Good candidates for result caching are frequently invoked functions that depend on relatively
static data.
 PL/SQL function code can specifythat results be cached.
Buffer Caches
A number of buffercaches are maintained in memoryin order to improve system responsetime.
Database Buffer Cache
The Database Buffer Cache is a fairly large memoryobjectthat stores the actual data blocks that are
retrieved from datafiles by system queries and other data manipulation language commands.
The purpose is to optimize physical input/output of data.
When DatabaseSmartFlash Cache (flash cache)is enabled,part of the buffercache can reside in
the flash cache.
 This buffercache extension is stored on a flash disk device,which is a solid state storage device
that uses flash memory.
 The database can improve performance bycaching buffers in flash memoryinstead of reading
from magnetic disk.
 Database Smart Flash Cache is available only in Solaris and Oracle Enterprise Linux.
A query causes a Server Process to look for data.

 The first look is in the Database BufferCache to determine if the requested informationhappens
to already be located in memory – thus the information would not need to be retrieved from disk
and this would speed up performance.
 If the information is not in the Database BufferCache, the Server Process retrieves the
information from disk and stores it to the cache.
 Keep in mind that information read from disk is read a block at a time, NOT a row at a time,
because a database block is the smallest addressable storagespace on disk.
Database blocks are kept in the Database BufferCache according to a LeastRecently Used (LRU)
algorithm and are aged out of memory if a buffercache block is not used in order to provide space for
the insertion of newly needed database blocks.
There are three bufferstates:
 Unused - a bufferis available for use - it has never been used or is currently unused.
 Clean - a bufferthat was used earlier - the data has been written to disk.
 Dirty - a bufferthat has modifieddata that has not beenwritten to disk.
Each bufferhas one of two access modes:
 Pinned - a bufferis pinned so it does not age out of memory.
 Free (unpinned).
The buffers in the cache are organized in two lists:
 the write list and,
 the least recently used (LRU) list.
The write list (also called a write queue)holds dirty buffers – these are buffers that hold that data that
has been modified,but the blocks have not been written back to disk.

The LRU list holds unused, free clean buffers,pinned buffers,and free dirty buffers that have not yet
been moved to the write list. Free clean buffers do not contain any useful data and are available for
use. Pinned buffers are currently being accessed.
When an Oracle process accesses a buffer,the processmoves the bufferto the most recently used
(MRU) end of the LRU list – this causes dirty buffers to age toward the LRU end of the LRU list.
When an Oracle user processneeds a data row, it searches for the data in the database buffercache
because memorycan be searched more quickly than hard disk can be accessed. If the data row is
already in the cache (a cache hit), the processreads the data from memory;otherwise a cache
missoccurs and data must be read from hard disk into the database buffercache.
Before reading a data block into the cache, the process must first find a free buffer.The process
searches the LRU list, starting at the LRU end of the list. The search continues until a free bufferis
found or until the search reaches the threshold limit of buffers.
Each time a user process finds a dirty bufferas it searches the LRU, that bufferis moved to the write list
and the search for a free buffercontinues.
When a user process findsa free buffer,it reads the data block from disk into the bufferand moves the
bufferto the MRU end of the LRU list.
If an Oracle user process searches the threshold limit of buffers without finding a free buffer,the process
stops searching the LRU list and signals the DBWnbackground process to write some of the dirty
buffers to disk. This frees up some buffers.
Database Buffer Cache Block Size

The block size for a database is set when a database is created and is determined by the init.ora
parameter file parameter named DB_BLOCK_SIZE.
 Typical blocksizes are 2KB, 4KB, 8KB, 16KB,and 32KB.
 The size of blocks in the Database BufferCache matches the block size for the database.
 The DBORCL database uses an 8KB block size.
 This figure shows that the use of non-standard blocksizes results in multiple database buffer
cache memory allocations.
Because tablespaces that store oracle tables can use different(non-standard) blocksizes, there can be
more than one Database BufferCache allocated to match blocksizes in the cache with the block sizes
in the non-standard tablespaces.

The size of the Database BufferCaches can be controlled by the
parameters DB_CACHE_SIZE and DB_nK_CACHE_SIZE to dynamically change the memoryallocated
to the caches without restarting the Oracle instance.
You can dynamically change the size of the Database BufferCache with the ALTER SYSTEMcommand
like the one shown here:
ALTER SYSTEM SET DB_CACHE_SIZE = 96M;
You can have the Oracle Server gather statistics about the Database BufferCache to help you size it to
achieve an optimal workload for the memory allocation. This information is displayed from
the V$DB_CACHE_ADVICE view. In order for statistics to be gathered, you can dynamically alter the
system by using the ALTER SYSTEM SET DB_CACHE_ADVICE (OFF,ON,
READY) command. However, gathering statistics on system performance always incurs some
overhead that will slow down system performance.
SQL> ALTER SYSTEM SET db_cache_advice = ON;
System altered.
SQL> DESC V$DB_cache_advice;
Name Null? Type
----------------------------------------- -------- -------------
ID NUMBER
NAME VARCHAR2(20)
BLOCK_SIZE NUMBER
ADVICE_STATUS VARCHAR2(3)
SIZE_FOR_ESTIMATE NUMBER
SIZE_FACTOR NUMBER

BUFFERS_FOR_ESTIMATE NUMBER
ESTD_PHYSICAL_READ_FACTOR NUMBER
ESTD_PHYSICAL_READS NUMBER
ESTD_PHYSICAL_READ_TIME NUMBER
ESTD_PCT_OF_DB_TIME_FOR_READS NUMBER
ESTD_CLUSTER_READS NUMBER
ESTD_CLUSTER_READ_TIME NUMBER
SQL> SELECT name, block_size, advice_status FROM v$db_cache_advice;
NAME BLOCK_SIZE ADV
-------------------- ---------- ---
DEFAULT 8192 ON
<more rows will display>
21 rows selected.
SQL> ALTER SYSTEM SET db_cache_advice = OFF;
System altered.
KEEP Buffer Pool
This pool retains blocks in memory (data from tables) that are likely to be reused throughout daily
processing. An example might be a table containing user names and passwords or a validation table of
some type.
The DB_KEEP_CACHE_SIZE parameter sizes the KEEP BufferPool.

RECYCLE Buffer Pool
This pool is used to store table data that is unlikely to be reused throughout daily processing – thus the
data blocks are quickly removed from memorywhen not needed.
The DB_RECYCLE_CACHE_SIZE parametersizes the Recycle BufferPool.

The Redo Log Buffer memoryobjectstores images of all changes made to database blocks.
 Database blocks typically store several table rows of organizational data. This means that if a
single column value from one row in a blockis changed, the block image is stored. Changes
include INSERT,UPDATE,DELETE,CREATE,ALTER,or DROP.
 LGWR writes redo sequentially to disk while DBWn performs scattered writes of data blocks to
disk.
o Scattered writes tend to be much slower than sequential writes.
o Because LGWR enable users to avoid waiting for DBWn to complete its slow writes, the
database delivers better performance.
The Redo Log Bufferas a circular bufferthat is reused over and over. As the bufferfills up, copies of
the images are stored to the Redo Log Files that are covered in more detail in a later module.
Large Pool
The Large Pool is an optional memory structure that primarily relieves the memoryburden placed on
the Shared Pool. The Large Poolis used for the following tasks if it is allocated:
 Allocating space for sessionmemory requirements from the User Global Area where a Shared
Server is in use.
 Transactions that interact with more than one database, e.g., a distributed database scenario.
 Backup and restore operations by the RecoveryManager (RMAN) process.
o RMAN uses this only if the BACKUP_DISK_IO = n and BACKUP_TAPE_IO_SLAVE=
TRUE parameters are set.
o If the Large Poolis too small, memory allocation for backup will fail and memory will be
allocated from the Shared Pool.

 Parallel execution message buffers forparallel server
operations. The PARALLEL_AUTOMATIC_TUNING = TRUE parameter must be set.
The Large Pool size is set with the LARGE_POOL_SIZE parameter – this is not a dynamic
parameter. It does not use an LRU list to manage memory.
Java Pool
The Java Pool is an optional memory object,but is required if the database has Oracle Java installed
and in use for Oracle JVM (Java Virtual Machine).
 The size is set with the JAVA_POOL_SIZE parameter that defaults to 24MB.
 The Java Pool is used for memory allocation to parse Java commands and to store data
associated with Java commands.
 Storing Java code and data in the Java Pool is analogous to SQL and PL/SQL code cached in the
Shared Pool.
Streams Pool
This pool stores data and control structures to support the Oracle Streams feature of Oracle Enterprise
Edition.
 Oracle Steams manages sharing of data and events in a distributed environment.
 It is sized with the parameter STREAMS_POOL_SIZE.
 If STEAMS_POOL_SIZE is not set or is zero, the size of the poolgrows dynamically.

Processes
You need to understand three differenttypes of Processes:
 User Process: Starts when a database user requests to connectto an Oracle Server.
 Server Process: Establishes the Connectionto an Oracle Instance when a User Process
requests connection– makes the connection for the User Process.
 BackgroundProcesses: These start when an Oracle Instance is started up.
Client Process
In order to use Oracle, you must connect to the database. This must occur whether you're using
SQLPlus,an Oracle tool such as Designeror Forms,or an application program. The client process is
also termed the user process in some Oracle documentation.

This generates a User Process (amemory object)that generates programmatic calls through your user
interface (SQLPlus,Integrated DeveloperSuite, or application program) that creates a sessionand
causes the generation of a Server Process that is either dedicated or shared.

Server Process
A Server Processis the go-betweenfor a Client Processand the Oracle Instance.
 Dedicated Server environment – there is a single Server Process to serve each Client Process.
 Shared Server environment – a Server Process can serve several User Processes,although with
some performance reduction.
 Allocation of server process in a dedicated environment versus a shared environment is covered
in further detail in the Oracle11g DatabasePerformanceTuning course offered by Oracle
Education.

Background Processes
As is shown here, there are both mandatory, optional, and slave background processesthat are started
whenever an Oracle Instance starts up. These background processesserve all system users. We will
cover mandatory process in detail.
Mandatory Background Processes
 ProcessMonitor Process (PMON)
 System Monitor Process(SMON)
 Database Writer Process (DBWn)
 Log Writer Process (LGWR)
 CheckpointProcess (CKPT)
 Manageability Monitor Processes(MMONand MMNL)
 Recover Process (RECO)
OptionalProcesses
 Archiver Process(ARCn)
 CoordinatorJob Queue (CJQ0)
 Dispatcher(number “nnn”)(Dnnn)
 Others
This query will display all background processes running to serve a database:
SELECT PNAME
FROM V$PROCESS
WHERE PNAME IS NOT NULL
ORDER BY PNAME;

PMON
The ProcessMonitor (PMON) monitors other background processes.
 It is a cleanup type of process that cleans up after failed processes.
 Examples include the dropping of a user connectiondue to a network failure or the abnormal
termination (ABEND) of a user application program.
 It cleans up the database buffercache and releases resources that were used by a failed user
process.
 It does the tasks shown in the figure below.

SMON
The System Monitor (SMON) does system-levelcleanup duties.
 It is responsible forinstance recovery by applying entries in the online redo log files to the
datafiles.
 Other processescan call SMON when it is needed.
 It also performs other activities as outlined in the figure shown below.

If an Oracle Instance fails, all information in memory not written to disk is lost. SMON is responsible for
recovering the instance when the database is started up again. It does the following:
 Rolls forward to recover data that was recorded in a Redo Log File, but that had not yet been
recorded to a datafile by DBWn. SMON reads the Redo Log Files and applies the changes to the
data blocks. This recovers all transactions that were committed because these were written to the
Redo Log Files prior to system failure.
 Opens the database to allow system users to logon.
 Rolls back uncommitted transactions.
SMON also does limited space management. It combines (coalesces)adjacent areas of free space in
the database's datafiles for tablespaces that are dictionary managed.
It also deallocates temporary segments to create free space in the datafiles.
DBWn (also called DBWR in earlier Oracle Versions)
The Database Writer writes modifiedblocks from the database buffercache to the datafiles.
 One database writer process(DBW0)is sufficientfor most systems.
 A DBA can configure up to 20 DBWnprocesses (DBW0through DBW9 and DBWathrough DBWj)
in order to improve write performance fora system that modifiesdata heavily.
 The initialization parameter DB_WRITER_PROCESSES specifiesthe number of DBWn processes.
The purpose of DBWn is to improve system performance by caching writes of database blocks from
the DatabaseBuffer Cache backto datafiles.
 Blocks that have been modified and that need to be written back to disk are termed "dirty
blocks."

 The DBWnalso ensures that there are enough free buffers in the Database BufferCache to
service Server Processes that may be reading data from datafiles into the Database Buffer
Cache.
 Performance improves because by delaying writing changed database blocks back to disk, a
Server Process may find the data that is needed to meeta User Process requestalready residing
in memory!
 DBWnwrites to datafiles when one of these events occurs that is illustrated in the figure below.

LGWR
The Log Writer (LGWR) writes contents from the Redo Log Bufferto the Redo Log File that is in use.
 These are sequential writes since the Redo Log Files record database modifications based onthe
actual time that the modificationtakes place.
 LGWR actually writes before the DBWnwrites and only confirms that a COMMIT operation has
succeeded whenthe Redo Log Buffer contents are successfully written to disk.
 LGWR can also call the DBWnto write contents of the Database BufferCache to disk.
 The LGWR writes according to the events illustrated in the figure shown below.

CKPT
The Checkpoint (CPT)process writes information to update the database control files and headers of
datafiles.
 A checkpointidentifies a point in time with regard to the Redo Log Files where instance recovery
is to begin should it be necessary.
 It can tell DBWnto write blocks to disk.
 A checkpointis taken at a minimum, once every three seconds.

Think of a checkpoint record as a starting point for recovery. DBWnwill have completed writing all
buffers from the Database BufferCache to disk prior to the checkpoint, thus those records will not
require recovery. This does the following:
 Ensures modified data blocks in memory are regularly written to disk – CKPT can call the DBWn
process inorder to ensure this and does so when writing a checkpointrecord.
 Reduces Instance Recoverytime by minimizing the amount of work needed forrecovery since
only Redo Log File entries processed sincethe last checkpointrequire recovery.
 Causes all committed data to be written to datafiles during database shutdown.

If a Redo Log File fills up and a switch is made to a new Redo Log File (this is covered in more detail in
a later module), the CKPT process also writes checkpoint information into the headers of the datafiles.

Checkpoint information written to control files includes the system change number (the SCN is a number
stored in the control file and in the headers of the database files that are used to ensure that all files in
the system are synchronized), location of which Redo Log File is to be used for recovery, and other
information.
CKPT does not write data blocks or redo blocks to disk – it calls DBWn and LGWR as necessary.
MMON and MMNL
The Manageability Monitor Process (MMNO) performstasks related to the Automatic Workload
Repository (AWR) – a repositoryof statistical data in the SYSAUX tablespace (see figure below) – for
example, MMON writes when a metric violates its threshold value, taking snapshots, and capturing
statistics value for recently modified SQL objects.

The Manageability Monitor Lite Process (MMNL) writes statistics from the Active SessionHistory
(ASH) bufferin the SGA to disk. MMNL writes to disk when the ASH bufferis full.
The information stored by these processesis used for performance tuning – we survey performance
tuning in a later module.

RECO
The Recoverer Process (RECO) is used to resolve failures of distributed transactions in a distributed
database.
 Considera database that is distributed on two servers – one in St. Louis and one in Chicago.
 Further, the database may be distributed on servers of two differentoperating systems,e.g.
LINUX and Windows.
 The RECO processof a node automatically connects to other databases involved in an in-doubt
distributed transaction.
 When RECO reestablishes a connection between the databases,it automatically resolves all in-
doubt transactions, removing from each database's pending transaction table any rows that
correspond to the resolved transactions.
Optional Background Processes
Optional Background ProcessDefinition:
 ARCn: Archiver – One or more archiver processes copythe online redo log files to archival storage
when they are full or a log switch occurs.
 CJQ0: CoordinatorJob Queue – This is the coordinatorof job queue processesforan instance. It
monitors the JOB$ table (table of jobs in the job queue) and starts job queue processes(Jnnn)as
needed to execute jobs The Jnnn processes execute job requests created by the DBMS_JOBS
package.
 Dnnn: Dispatchernumber "nnn", for example, D000 would be the first dispatcherprocess –
Dispatchers are optional background processes,presentonly when the shared server configuration is
used. Shared server is discussedin your readings on the topic "Configuring Oracle for the Shared
Server".

 FBDA: Flashback Data Archiver Process – This archives historical rows of tracked tables into
Flashback Data Archives. When a transaction containing DML on a tracked table commits,this
process stores the pre-image of the rows into the Flashback Data Archive. It also keeps metadata on
the current rows. FBDA automatically manages the flashback data archive for space,organization,
and retention
Of these, you will most often use ARCn (archiver) when you automatically archive redo log file
information (covered in a later module).
ARCn
While the Archiver (ARCn) is an optional background process, we cover it in more detail because it is
almost always used for production systems storing mission critical information.
 The ARCn processmust be used to recover from loss of a physical disk drive forsystems that are
"busy" with lots of transactions being completed.
 It performs the tasks listed below.

When a Redo Log File fills up, Oracle switches to the next Redo Log File.
 The DBA creates several of these and the details of creating them are covered in a later module.
 If all Redo Log Files fill up, then Oracle switches back to the first one and uses them in a round-
robin fashion by overwriting ones that have already beenused.
 Overwritten Redo Log Files have information that, once overwritten, is lost forever.
ARCHIVELOG Mode:
 If ARCn is in what is termed ARCHIVELOG mode,then as the Redo Log Files fill up, they are
individually written to Archived Redo Log Files.
 LGWR does not overwrite a Redo Log File until archiving has completed.
 Committed data is not lost forever and can be recovered in the event of a disk failure.
 Only the contents of the SGA will be lost if an Instance fails.

In NOARCHIVELOG Mode:
 The Redo Log Files are overwritten and not archived.
 Recoverycan only be made to the last full backup of the database files.
 All committed transactions after the last full backup are lost, and you can see that this could cost
the firm a lot of $$$.
When running in ARCHIVELOG mode,the DBA is responsible to ensure that the Archived Redo Log
Files do not consume all available disk space! Usually after two completebackups are made, any
Archived Redo Log Files for prior backups are deleted.
Slave Processes
Slave processes are background processes that perform work on behalf of other processes.
Innn: I/O slave processes-- simulate asynchronous I/O for systems and devices that do not supportit.
In asynchronous I/O,there is no timing requirement for transmission, enabling other processes to start
before the transmission has finished.
 For example, assume that an application writes 1000 blocks to a disk on an operating system that
does not supportasynchronous I/O.
 Each write occurs sequentially and waits for a confirmation that the write was successful.
 With asynchronous disk, the application can write the blocks in bulk and perform other work while
waiting for a response from the operating system that all blocks were written.
ParallelQuery Slaves-- In parallel execution orparallel processing,multiple processeswork
together simultaneously to run a single SQL statement.
 By dividing the work among multiple processes,Oracle Database can run the statement more
quickly.

 For example, four processeshandle four differentquarters in a year instead of one process
handling all four quarters by itself.
 Parallel execution reduces response time fordata-intensive operations on large databases such
as data warehouses. Symmetric multiprocessing (SMP) and clustered system gain the largest
performance benefits from parallel execution because statement processing can be split up among
multiple CPUs. Parallel execution can also benefitcertain types of OLTP and hybrid systems.

Logical Structure
It is helpful to understand how an Oracle database is organized in terms of a logical structure that is
used to organize physical objects.
Tablespace: An Oracle database must always consist of at least
two tablespaces(SYSTEM and SYSAUX), although a typical Oracle database will multiple
tablespaces.

 A tablespace is a logical storage facility (a logical container) for storing objects suchas tables,
indexes, sequences,clusters,and other database objects.
 Each tablespace has at least one physical datafile that actually stores the tablespace at the
operating system level. A large tablespace may have more than one datafile allocated for storing
objects assigned to that tablespace.
 A tablespace belongs to only one database.
 Tablespacescan be brought online and taken offline for purposes of backup and management,
exceptfor the SYSTEM tablespace that must always be online.
 Tablespacescan be in either read-only or read-write status.
Datafile: Tablespacesare stored in datafiles which are physical disk objects.
 A datafile can only store objects fora single tablespace,but a tablespace may have more than
one datafile – this happens when a disk drive device fills up and a tablespace needs to be
expanded,then it is expanded to a new disk drive.
 The DBA can change the size of a datafile to make it smaller or later. The file can also grow in
size dynamically as the tablespace grows.
Segment: Whenlogical storage objects are created within a tablespace,forexample, an employee
table, a segmentis allocated to the object.
 Obviously a tablespace typically has many segments.
 A segmentcannot span tablespaces butcan span datafiles that belong to a single tablespace.
Extent: Each objecthas one segmentwhich is a physical collectionof extents.
 Extents are simply collections of contiguousdisk storageblocks. A logical storage objectsuch
as a table or index always consists of at least one extent – ideally the initial extent allocated to an
objectwill be large enough to store all data that is initially loaded.
 As a table or index grows, additional extents are added to the segment.
 A DBA can add extents to segments in order to tune performance of the system.

 An extent cannot span a datafile.
Block: The Oracle Server manages data at the smallestunit in what is termed a block or data
block. Data are actually stored in blocks.
A physical block is the smallest addressable location on a disk drive for read/write operations.

An Oracle data block consists of one or more physical blocks (operating system blocks)so the data
block, if larger than an operating system block, should be an even multiple of the operating system block
size, e.g., if the Linux operating system block size is 2K or 4K, then the Oracle data block should be 2K,
4K, 8K, 16K, etc in size. This optimizes I/O.
The data block size is set at the time the database is created and cannot be changed. It is set with
the DB_BLOCK_SIZE parameter. The maximum data blocksize depends on the operating system.
Thus, the Oracle database architecture includes both logical and physical structures as follows:
 Physical: Control files;Redo Log Files;Datafiles; Operating System Blocks.
 Logical: Tablespaces;Segments;Extents; Data Blocks.
SQL Statement Processing
SQL Statements are processed differentlydepending on whether the statement is a query, data
manipulation language (DML)to update, insert, or delete a row, or data definition language (DDL) to
write information to the data dictionary.

Processing a query:
 Parse:
o Search for identical statement in the Shared SQL Area.
o Check syntax, objectnames, and privileges.
o Lock objects used during parse.
o Create and store execution plan.
 Bind: Obtains values for variables.
 Execute: Process statement.
 Fetch: Return rows to user process.
Processing a DML statement:
 Parse: Same as the parse phase used for processing aquery.

 Bind: Same as the bind phase used for processing a query.
 Execute:
o If the data and undo blocks are not already in the Database BufferCache, the server process
reads them from the datafiles into the Database BufferCache.
o The server processplaces locks on the rows that are to be modified.The undo block is used
to store the before image of the data, so that the DML statements can be rolled back if
necessary.
o The data blocks record the new values of the data.
o The server processrecords the before image to the undo blockand updates the data
block. Both of these changes are made in the Database BufferCache. Any changed blocks
in the Database BufferCache are marked as dirty buffers. That is, buffers that are not the
same as the corresponding blocks on the disk.
o The processing of a DELETE or INSERT command uses similar steps. The before image for
a DELETEcontains the column values in the deleted row, and the before image of an
INSERT contains the row location information.
Processing a DDL statement:
 The execution of DDL (Data Definition Language) statements differs from the execution of DML
(Data Manipulation Language) statements and queries, because the success ofa DDL statement
requires write access to the data dictionary.
 For these statements, parsing actually includes parsing, data dictionary lookup, and
execution. Transaction management, sessionmanagement, and system management SQL
statements are processed using the parse and execute stages. To re-execute them, simply
perform another execute.

Oracle architecture

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