Oracle Architecture Notes

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 different types of
files: datafiles, control files, redo log files and archive redo log
files. The Oracle server also accesses parameter 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 different sets of

components:
• The first component set is the set of background
processes (PMON, SMON, RECO, DBW0, LGWR, CKPT, D000 and
others).
o These will be covered later in detail – each background process
is a computer program.
o These processes perform input/output and monitor other Oracle
processes to provide good performance and database
reliability.
• The second component set includes the memory structures 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 processes also 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 recover the database in the event of
application program failures, instance failures and other minor
failures.
• The archived redo log files are used to recover the database if a
disk fails.
• Other files not shown in the figure include:
o The required parameter file that is used to specify parameters
for configuring an Oracle instance when it starts up.
o The optional password file authenticates special users of the
database – these are termed privileged users and include
database administrators.
o Alert and Trace Log Files – these files store information about
errors and actions taken that affect the configuration of the
database.

User and server processes: The processes shown in the figure are
called user and server processes. These processes are used to manage
the execution of SQL statements.
• A Shared Server Process can share memory and variable
processing for multiple user processes.
• A Dedicated Server Process manages memory and variables for a
single user process.

Connecting to an Oracle Instance – Creating a

Session
System users can connect to an Oracle database through SQLPlus or
through an application program like the Internet Developer Suite (the
program becomes the system user). This connection enables 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 communicates with 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 between the 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 Process shares a Server Process.

Sessions: When a user connects to an Oracle server, this is termed a

session. The User Global Area is session memory and these memory
structures are described later in this document. The session starts when
the Oracle server validates the user for connection. The session ends
when the user logs out (disconnects) or if the connection terminates
abnormally (network failure or client computer failure).

A user can typically have more than one concurrent session, e.g., the user
may connect using SQLPlus and also connect using Internet Developer
Suite 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.
• Control files – 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 specify how the
database instance is to start up. For example, some
parameters will specify how 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 specify how 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 necessary for recovery in an online, transaction-processing
environment in the event of a disk failure.
Memory Structure
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

This figure from the Oracle Database Administration Guide provides
another way of viewing the SGA.
The SGA is a read/write memory area that stores information shared by all
database processes and by all users of the database (sometimes it is
called theShared Global Area).
o This information includes both organizational data and control
information used by the Oracle Server.
o The SGA is allocated in memory and 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 specified in the initialization parameter file (either PFILE or
SPFILE).

The SGA has the following mandatory memory structures:

• Database Buffer Cache
• 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 memory structures in the SGA include:

• Large Pool

Early versions of Oracle used a Static SGA. This meant that if

modifications to memory management 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 9i, 10g, and 11g use a Dynamic SGA. Memory configurations for
the system global area can be made without shutting down the database
instance. The advantage is obvious. This allows the DBA to resize the
Database Buffer Cache and Shared Pool dynamically.
Several initialization parameters are set that affect the amount of random
access memory dedicated 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_SIZE in
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 memory allocated to the Database Buffer Cache 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 specifies the
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 parameter
PARALLEL_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, L
ARGE_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 on the SGA_MAX_SIZE
parameter. 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 Buffer Cache, Shared Pool, Java
Pool, and other memory structures, and these memory components 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.

Program Global Area

 The Program Global Area is also termed the Process Global Area
(PGA) and is a part of memory allocated that is outside of the Oracle
Instance.
• It stores data and control information for a single Server Process or
a single Background Process.
• It is allocated when a process is 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 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
session issuing 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 often happens in OLTP
environments where many users are executing and using the same
application program.
o Dedicated Server environment – the Private SQL Area is located
in the Program Global Area.
o Shared Server environment – the Private SQL Area is located in
the System Global Area.

• Session Memory: Memory that holds session variables and other

session information.

• 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
Work Areas 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 Global Area is session memory.

A session that loads a PL/SQL package into memory has the package
state stored to the UGA. The package state 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 session and released at the end of
the session. An OLAP session opens automatically whenever a user
queries a dimensional object such as a cube.

Note: Oracle OLAP is a multidimensional analytic engine embedded

in Oracle Database 11g. Oracle OLAP cubes deliver sophisticated
calculations using simple SQL queries - producing results with speed
of thought response times.

The UGA must be available to a database session for the 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 specific to a single
process. Therefore, the UGA is stored in the SGA when using shared
server connections, enabling any shared server process access to 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 specify SGA Component sizes
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 specify the

total SGA memory available through the SGA_TARGET initialization
parameter. The Oracle Database automatically distributes this memory
among various subcomponents to ensure most effective memory utilization.

The DBORCL database SGA_TARGET is set in the initDBORCL.ora file:

sga_target=1610612736

With automatic SGA memory management, the different SGA 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 forget about the sizes of individual components. No out of memory
errors are generated unless the system has actually run out of
memory. No manual tuning effort is needed.

The SGA_TARGET initialization parameter reflects the total size of the

SGA and includes memory for 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 buffer cache
 • The keep and recycle buffer caches (if specified)
• Nonstandard block size buffer caches (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

10g 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 buffer cache

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/Recycle buffer caches (controlled
by DB_KEEP_CACHE_SIZE and DB_RECYCLE_CACHE_SIZE)
• Additional buffer caches 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
component can be viewed in the view V$SGAINFO. The size of each
component and the time and type of the last resize operation performed on
each component can be viewed in the
view V$SGA_DYNAMIC_COMPONENTS.
Shared Pool

The Shared Pool is a memory structure 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
dictionary information.
• The shared pool is involved in almost every operation that occurs in
the database. For example, if a user executes a SQL statement, then
Oracle Database accesses the shared pool.
• It consists of both fixed and variable structures.
• The variable component grows 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-bit platforms.
Increasing the value of this parameter increases the amount of memory
reserved for the shared pool.

You can alter the size of the shared pool dynamically with the ALTER
SYSTEM SET command. An example command is shown in the figure
below. You must keep in mind that the total memory allocated 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 Pool is 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 Pool stores the most recently 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 Pool includes 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 most
recently 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 oldest and least recently used are purged
when more storage space is needed.

The Library Cache is composed of two memory subcomponents:

• 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 session issuing 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 different client processes issuing the
same SQL statement – the parsed solution is already in the
Shared SQL Area.

Data Dictionary Cache

The Data Dictionary Cache is a memory structure that caches data

dictionary information that has been recently used.
 • This cache is necessary because the data dictionary is accessed so
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 Pool in which the
Data Dictionary Cache resides. If the size is too small, then the data
dictionary tables that reside on disk must be queried often for 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
statement repeatedly. 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 specify that results be cached.
Buffer Caches
A number of buffer caches are maintained in memory in order to improve
system response time.

Database Buffer Cache

The Database Buffer Cache is a fairly large memory object that stores the
actual data blocks that are retrieved from datafiles by system queries and
other data manipulation language commands.

A query causes a Server Process to first look in the Database Buffer

Cache to determine if the requested information happens 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 Buffer Cache, 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
storage space on disk.

Database blocks are kept in the Database Buffer Cache according to

a Least Recently Used (LRU) algorithm and are aged out of memory if a
buffer cache block is not used in order to provide space for the insertion of
newly needed database blocks.

The buffers in the cache are organized in two lists:

• the write list and,
• the least recently used (LRU) list.

The write list 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 free buffers, pinned buffers, and dirty buffers that have
not yet been moved to the write list. Free 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 process moves the buffer
to 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 process needs a data row, it searches for the data in
the database buffer cache because memory can be searched more quickly
than hard disk can be accessed. If the data row is already in the cache
(a cache hit), the process reads the data from memory; otherwise a cache
miss occurs and data must be read from hard disk into the database buffer
cache.

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 buffer is found or until the search
reaches the threshold limit of buffers.

Each time a user process finds a dirty buffer as it searches the LRU, that
buffer is moved to the write list and the search for a free buffer continues.

When a user process finds a free buffer, it reads the data block from disk
into the buffer and moves the buffer to 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 DBWn background process to write some of the dirty buffers to
disk. This frees up some buffers.

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 block sizes are 2KB, 4KB, 8KB, 16KB, and 32KB.
• The size of blocks in the Database Buffer Cache matches the block
size for the database.
• The DBORCL database uses an 8KB block size.
• This figure shows that the use of non-standard block sizes results in
multiple database buffer cache memory allocations.
Because tablespaces that store oracle tables can use different (non-
standard) block sizes, there can be more than one Database Buffer Cache
allocated to match block sizes in the cache with the block sizes in the non-
standard tablespaces.

The size of the Database Buffer Caches can be controlled by the

parameters DB_CACHE_SIZE and DB_nK_CACHE_SIZE to dynamically
change the memory allocated to the caches without restarting the Oracle
instance.

You can dynamically change the size of the Database Buffer Cache with
the ALTER SYSTEM command like the one shown here:

ALTER SYSTEM SET DB_CACHE_SIZE = 96M;

You can have the Oracle Server gather statistics about the Database Buffer
Cache 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? Typ
e
----------------------------------------- -------- ---
----------
ID NUM
BER
NAME VAR
CHAR2(20)
BLOCK_SIZE NUM
BER
ADVICE_STATUS VAR
CHAR2(3)
SIZE_FOR_ESTIMATE NUM
BER
SIZE_FACTOR NUM
BER
BUFFERS_FOR_ESTIMATE NUM
BER
ESTD_PHYSICAL_READ_FACTOR NUM
BER
ESTD_PHYSICAL_READS NUM
BER
ESTD_PHYSICAL_READ_TIME NUM
BER
ESTD_PCT_OF_DB_TIME_FOR_READS NUM
BER
ESTD_CLUSTER_READS NUM
BER
ESTD_CLUSTER_READ_TIME NUM
BER

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 Buffer Pool.

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 memory
when not needed.

The DB_RECYCLE_CACHE_SIZE parameter sizes the Recycle Buffer

Pool.
Redo Log Buffer

The Redo Log Buffer memory object stores 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
block is 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 Buffer as a circular buffer that is reused over and over. As
the buffer fills 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
memory burden placed on the Shared Pool. The Large Pool is used for the
following tasks if it is allocated:
• Allocating space for session memory 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 Recovery Manager (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 Pool is too small, memory allocation for backup will
fail and memory will be allocated from the Shared Pool.
• Parallel execution message buffers for parallel 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 pool
grows dynamically.

Processes
You need to understand three different types of Processes:
• User Process: Starts when a database user requests to connect to
an Oracle Server.
• Server Process: Establishes the Connection to an Oracle Instance
when a User Process requests connection – makes the connection
for the User Process.
• Background Processes: These start when an Oracle Instance is
started up.

Client Process
In order to use Oracle, you must obviously connect to the database. This
must occur whether you're using SQLPlus, an Oracle tool such as Designer
or Forms, or an application program. The client process is also termed the
user process in some Oracle documentation.
This generates a User Process (a memory object) that generates
programmatic calls through your user interface (SQLPlus, Integrated
Developer Suite, or application program) that creates a session and causes
the generation of a Server Process that is either dedicated or shared.

Server Process
A Server Process is the go-between for a Client Process and 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.

Background Processes
As is shown here, there are both mandatory and optional background
processes that are started whenever an Oracle Instance starts up. These
background processes serve all system users. We will cover mandatory
process in detail.

Mandatory Background Processes

• Process Monitor Process (PMON)
• System Monitor Process (SMON)
• Database Writer Process (DBWn)
• Log Writer Process (LGWR)
• Checkpoint Process (CKPT)
• Manageability Monitor Processes (MMON and MMNL)
• Recover Process (RECO)

Optional Processes
• Archiver Process (ARCn)
• Coordinator Job Queue (CJQ0)
• Dispatcher (number “nnn”) (Dnnn)
• Others

Optional Background Process Definition:

• ARCn: Archiver – One or more archiver processes copy the online redo
log files to archival storage when they are full or a log switch occurs.
• CJQ0: Coordinator Job Queue – This is the coordinator of job queue
processes for an 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: Dispatcher number "nnn", for example, D000 would be the first
dispatcher process – Dispatchers are optional background processes,
present only when the shared server configuration is used. Shared
server is discussed in your readings on the topic "Configuring Oracle for
the Shared Server".

Of these, you will most often use ARCn (archiver) when you automatically
archive redo log file information (covered in a later module).

PMON
The Process Monitor (PMON) is a cleanup type of process that cleans up
after failed processes such as the dropping of a user connection due to a
network failure or the abnormal termination (ABEND) of a user application
program. It does the tasks shown in the figure below.
SMON
The System Monitor (SMON) is responsible for instance recovery by
applying entries in the online redo log files to the datafiles. 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 modified blocks from the database buffer
cache to the datafiles. Although one database writer process (DBW0) is
sufficient for most systems, you can configure up to 20 DBWn processes
(DBW0 through DBW9 and DBWa through DBWj) in order to improve write
performance for a system that modifies data heavily.

The initialization parameter DB_WRITER_PROCESSES specifies the number of

DBWn processes.

The purpose of DBWn is to improve system performance by caching writes

of database blocks from the Database Buffer Cache back to
datafiles. Blocks that have been modified and that need to be written back
to disk are termed "dirty blocks." The DBWn also ensures that there are
enough free buffers in the Database Buffer Cache 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 meet a User Process request already residing in memory!

DBWn writes 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 Buffer to the
Redo Log File that is in use. These are sequential writes since the Redo
Log Files record database modifications based on the actual time that the
modification takes place. LGWR actually writes before the DBWn writes
and only confirms that a COMMIT operation has succeeded when the Redo
Log Buffer contents are successfully written to disk. LGWR can also call
the DBWn to write contents of the Database Buffer Cache 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 to identify the point in time with regard
to theRedo Log Files where instance recovery is to begin should it be
necessary. This is done at a minimum, once every three seconds.
Think of a checkpoint record as a starting point for recovery. DBWn will
have completed writing all buffers from the Database Buffer Cache 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 in order to ensure this and does
so when writing a checkpoint record.
• Reduces Instance Recovery time by minimizing the amount of work
needed for recovery since only Redo Log File entries processed
since the last checkpoint require 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) performs tasks related to the
Automatic Workload Repository (AWR) – a repository of 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 Session History (ASH) buffer in the SGA to disk. MMNL writes to
disk when the ASH buffer is full.
The information stored by these processes is 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.
• Consider a 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 different
operating systems, e.g. LINUX and Windows.
• The RECO process of 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.

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 process must be used to
recover from loss of a physical disk drive for systems that are "busy" with
lots of transactions being completed.
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 been used – it should be obvious that the
information stored on the files, once overwritten, is lost forever.

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 and
LGWR does not overwrite a Redo Log File until archiving has
completed. Thus, 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. Recovery can 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 complete backups are made, any Archived Redo
Log Files for prior backups are deleted.

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 such as 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.
• Tablespaces can be brought online and taken offline for purposes of
backup and management, except for the SYSTEM tablespace that
must always be online.
• Tablespaces can be in either read-only or read-write status.

Datafile: Tablespaces are stored in datafiles which are physical disk

objects.
• A datafile can only store objects for a 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: When logical storage objects are created within a tablespace,

for example, an employee table, a segment is allocated to the object.
• Obviously a tablespace typically has many segments.
• A segment cannot span tablespaces but can span datafiles that
belong to a single tablespace.

Extent: Each object has one segment which is a physical collection

of extents.
• Extents are simply collections of contiguous disk storage
blocks. A logical storage object such as a table or index always
consists of at least one extent – ideally the initial extent allocated to
an object will 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 smallest unit 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 block size 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 differently depending 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, object names, 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 a query.
• 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
Buffer Cache, the server process reads them from the datafiles
into the Database Buffer Cache.
o The server process places 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 process records the before image to the undo block
and updates the data block. Both of these changes are made
in the Database Buffer Cache. Any changed blocks in the
Database Buffer Cache 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 DELETE contains 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 of a DDL statement requires write
access to the data dictionary.
• For these statements, parsing actually includes parsing, data
dictionary lookup, and execution. Transaction management, session
management, and system management SQL statements are
processed using the parse and execute stages. To re-execute them,
simply perform another execute.