Database Systems
Lecture 5
Overview of Oracle Database Architecture
- Concept Manual : Chapters 1,8
Lecturer : Dr Bela Stantic
Database Systems
Slide 1
How Oracle Works
The following example is based on an Oracle configuration where the user and associated
server process are on separate machines (connected through a network).









An instance is started on the computer running Oracle (host /server).
A computer (client) runs an application (say SQLPLUS) in a user process. The
application attempts to establish a connection to the server.
The server detects the connection request from the application and creates a
dedicated server process on behalf of the user process.
The user runs a SQL statement and commits the transaction. For example, the user
changes a name in a row of a table.
The server process receives the statement and checks the user’s access privileges to
the requested data, so it can be parsed and processed.
The server process retrieves the necessary data values from the datafile or memory.
The server process modifies data in the system global area (SGA). The DBWn
process writes modified blocks permanently to disk when doing so is efficient.
Because the transaction is committed, the LGWR process immediately records the
transaction in the redo log file.
If the transaction is successful, then the server process sends a message across the
network to the application. If it is not successful, then an error message is transmitted.
Throughout this entire procedure, the other background processes run, watching for
conditions that require intervention. In addition, the database server manages other
users’ transactions and prevents contention between transactions that request the
same data.
Database Systems
Slide 2
Database Systems
Slide 3
Oracle Architecture – Key Components
Database Systems
Slide 4
Overview of Physical Database Structures
The physical database structures of an database consist of:
– Datafiles,
– Redo Log Files,
– Control Files,
– Other Files.
Datafiles
Every database has one or more physical datafiles. The datafiles contain all
the database data. The data of logical database structures, such as tables
and indexes, is physically stored in the datafiles allocated for a database.
The characteristics of datafiles are:
– A datafile can be associated with only one database.
– Datafiles can have certain characteristics set to let them automatically
extend when the database runs out of space.
– One or more datafiles form a logical unit of database storage called a
tablespace.
Database Systems
Slide 5
Control Files

Every database has a Control File. A control file contains entries
that specify the physical structure of the database. For example, it
contains the following information:
– Database name
– Names and locations of datafiles and redo log files
– Time stamp of database creation

Because control files are important we can multiplex them, to
protect against a failure involving the control file.

Every time an instance of a database is started, its control file
identifies the database and redo log files that must be opened for
database operation to proceed.

A control file is also used in database recovery.
Database Systems
Slide 6
Redo Log Files







A database has a set of two or more redo log files. A redo log is made up of
redo entries (also called redo records).
The primary function of the redo log is to record all changes made to data. If
a failure prevents modified data from being permanently written to the
datafiles, then the changes can be obtained from the redo log, so work is
never lost.
To protect against a failure involving the redo log itself, Oracle allows a
multiplexed redo log so that two or more copies of the redo log can be
maintained on different disks.
The information in a redo log file is used only to recover the database from a
system or media failure that prevents database data from being written to the
datafiles.
For example, if an unexpected power outage terminates database operation,
then data in memory cannot be written to the datafiles, and the data is lost.
However, lost data can be recovered when the database is opened, after
power is restored.
By applying the information in the most recent redo log files to the database
datafiles, Oracle restores the database to the time at which the power failure
occurred.
The process of applying the redo log during a recovery operation is called
rolling forward.
Database Systems
Slide 7
Other Files

Archive Log Files
You can enable automatic archiving of the redo log. Oracle automatically
archives log files when the database is in ARCHIVELOG mode.

Parameter Files
Parameter files contain a list of configuration parameters for that instance
and database.

Alert and Trace Log Files
Each server and background process can write to an associated trace file.
Some of the information written to a trace file is intended for the database
administrator, while other information is for Oracle Support Services.
– Trace file information is also used to tune applications and instances
– The alert file, or alert log, is a special trace file. The alert file of a
database is a chronological log of messages and errors

Backup Files
To restore a file is to replace it with a backup file. Typically, you restore a file
when a media failure or user error has damaged or deleted the original file.
Database Systems
Slide 8
Overview of Logical Database Structures
The logical storage structures, including data blocks, extents, and
segments, enable DBMS to have fine-grained control of disk space
use.

Tablespaces
– A database is divided into logical storage units called tablespaces,
which is the largest logical structure.
– Tablespaces commonly group together all application objects to
simplify some administrative operations.
– One or more datafiles are explicitly created for each tablespace to
physically store the data of all logical structures in a tablespace.
More about tablespaces will be covered in the lecture on Managing
Storage in Databases
Database Systems
Slide 9
Overview of Logical Database Structures … contd()

Oracle Data Blocks
At the finest level of granularity, database data are stored in data
blocks. One data block corresponds to a specific number of bytes
of physical database space on disk.

Extents
The next level of logical database space is an extent. An extent is a
specific number of contiguous data blocks, obtained in a single
allocation, used to store a specific type of information.

Segments
Above extents, the level of logical database storage is a segment. A
segment is a set of extents allocated for a certain logical structure.
Database Systems
Slide 10
Memory Architecture

RDBMS uses memory to store information such as the following:
– Program code
– Information about a connected session, even if it is not currently
active
– Information needed during program execution
– Information that is shared and communicated among other
processes (for example, locking information)
– Cached data that is also permanently stored on peripheral
memory (for example, data blocks and redo log entries)

The basic memory structures associated with Oracle include:
– System Global Area (SGA), which is shared by all server and
background processes.
– Program Global Areas (PGA), which is private to each server and
background process; there is one PGA for each process.
Database Systems
Slide 11
Memory Structures
Database Systems
Slide 12
Overview of the System Global Area







The System Global Area (SGA) is a group of shared memory structures
that contain data and control information for one Oracle database instance.
If multiple users are concurrently connected to the same instance, then the
data in the instance’s SGA is shared among the users.
The SGA and Oracle processes together constitute an Oracle instance.
Oracle automatically allocates memory for an SGA when you start an
instance.
Each database instance has its own SGA.
The SGA is read/write. All users connected to a multiple-process database
instance can read information contained within the instance’s SGA, and
several processes write to the SGA during execution of Oracle.
The SGA contains the following data structures:
– Database buffer cache
– Redo log buffer
– Shared pool
– Java pool
– Large pool (optional)
– Streams pool
– Data dictionary cache
– Other miscellaneous information
Database Systems
Slide 13
Database Buffer Cache

The Database Buffer Cache is the portion of the SGA that holds copies of data
blocks read from datafiles.

The buffers in the cache are organized in two lists:
– The write list (Also called dirty list)
– the least recently used (LRU) list.

The write list holds dirty buffers, which contain data that has been modified but
has not yet been written 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. As more buffers are
continually moved to the MRU end of the LRU list, dirty buffers age toward the
LRU end of the LRU list.
Database Systems
Slide 14
Database Buffer Cache … contd()

When user process requires a particular piece of data, it searches
for the data in the database buffer cache. If the process finds the
data already in the cache (a cache hit), it can read the data directly
from memory.

If the process cannot find the data in the cache (a cache miss), it
must copy the data block from a datafile on disk into a buffer in the
cache before accessing the data.

The LRU Algorithm and Full Table Scans
– When the user process is performing a full table scan, it reads the
blocks of the table into buffers and puts them on the LRU end
(instead of the MRU end) of the LRU list. This is because a fully
scanned table usually is needed only briefly, so the blocks should
be moved out quickly to leave more frequently used blocks in the
cache.
Database Systems
Slide 15
Memory Structures … contd()
SGA
Shared Pool
Redo Log
Database Buffer Cache
LGWR
DBWR
Server proc.
Redo Log Files
User proc.
Data Files
Database Systems
Slide 16
Redo Log Buffer

The Redo Log Buffer is a circular buffer in the SGA that holds
information about changes made to the database.

This information is stored in redo entries. Redo entries contain the
information necessary to reconstruct, or redo, changes made to the
database by INSERT, UPDATE, DELETE, CREATE, ALTER, or
DROP operations.

Redo entries are used for database recovery, if necessary.

Redo entries are copied by Oracle database processes from the
user’s memory space to the redo log buffer in the SGA.

The redo entries take up continuous, sequential space in the buffer.

The background process LGWR writes the redo log buffer to the
active redo log file (or group of files) on disk.
Database Systems
Slide 17
Shared Pool
The shared pool portion of the SGA contains following buffers for
parallel execution messages, and control structures:
– the library cache,
– the dictionary cache,
Library Cache
The library cache includes:
– the shared SQL areas,
– private SQL areas,
– PL/SQL procedures and packages,
– control structures such as locks and library cache handles.
Database Systems
Slide 18
Shared SQL Areas






Shared SQL areas are accessible to all users, so the library cache is
contained in the shared pool within the SGA.
Oracle represents each SQL statement it runs with a shared SQL area and a
private SQL area. Oracle recognizes when two users are executing the same
SQL statement and reuses the shared SQL area for those users. However,
each user must have a separate copy of the statement’s private SQL area.
A shared SQL area contains the parse tree and execution plan for a given
SQL statement. Oracle saves memory by using one shared SQL area for
SQL statements run multiple times, which often happens when many users
run the same application.
Oracle allocates memory from the shared pool when a new SQL statement is
parsed, to store in the shared SQL area. The size of this memory depends
on the complexity of the statement.
If the entire shared pool has already been allocated, Oracle can deallocate
items from the pool using a modified LRU (least recently used) algorithm
until there is enough free space for the new statement’s shared SQL area.
If Oracle deallocates a shared SQL area, the associated SQL statement
must be reparsed and reassigned to another shared SQL area at its next
execution.
Database Systems
Slide 19
Private SQL Area






A private SQL area contains data such as bind information and runtime
memory structures.
Each session that issues a SQL statement has a private SQL area.
Each user that submits the same SQL statement has his or her own private
SQL area that uses a single shared SQL area. Thus, many private SQL
areas can be associated with the same shared SQL area.
The private SQL area of a cursor is itself divided into two areas whose
lifetimes are different:
– The persistent area, which contains, for example, bind information. It is
freed only when the cursor is closed.
– The run-time area, which is freed when the execution is terminated.
Oracle creates the runtime area as the first step of an execute request. For
INSERT, UPDATE, and DELETE statements, Oracle frees the runtime area
after the statement has been run. For queries, Oracle frees the runtime area
only after all rows are fetched or the query is cancelled.
If a session is connected through a dedicated server, private SQL areas are
located in the server process’s PGA. However, if a session is connected
through a shared server, part of the private SQL area is kept in the SGA.
Database Systems
Slide 20
PL/SQL Program Units and the Shared Pool





Oracle processes PL/SQL program units (procedures, functions,
packages, anonymous blocks, and database triggers) much the
same way it processes individual SQL statements.
Oracle allocates a shared area to hold the parsed, compiled form of
a program unit.
Oracle allocates a private area to hold values specific to the session
that runs the program unit, including local, global, and package
variables (also known as package instantiation) and buffers for
executing SQL.
If more than one user runs the same program unit, then a single,
shared area is used by all users, while each user maintains a
separate copy of his or her private SQL area, holding values specific
to his or her session.
Despite their origins within a PL/SQL program unit, these SQL
statements use a shared area to hold their parsed representations
and a private area for each session that runs the statement.
Database Systems
Slide 21
Dictionary Cache





The data dictionary is a collection of database tables and views
containing reference information about the database, its structures,
and its users.
Oracle accesses the data dictionary frequently during SQL statement
parsing. This access is essential to the continuing operation of
Oracle.
The data dictionary is accessed so often by Oracle that two special
locations in memory are designated to hold dictionary data.
One area is called the Data Dictionary Cache, also known as the
row cache because it holds data as rows instead of buffers (which
hold entire blocks of data).
The other area in memory to hold dictionary data is the Library
Cache. All Oracle user processes share these two caches for access
to data dictionary information.
Database Systems
Slide 22
Large Pool





The database administrator can configure an optional memory area
called the Large Pool to provide large memory allocations for:
– Session memory for the shared server
– I/O server processes,
– Oracle backup and restore operations
By allocating session memory from the Large Pool for shared server,
Oracle can use the Shared Pool primarily for caching shared SQL
and avoid the performance overhead caused by shrinking the shared
SQL cache.
In addition, the memory for Oracle backup and restore operations, for
I/O server processes, and for parallel buffers is allocated in buffers of
a few hundred kilobytes.
The Large Pool is better able to satisfy such large memory requests
than the Shared Pool.
The Large Pool does not have an LRU list. It is different from
reserved space in the Shared Pool, which uses the same LRU list as
other memory allocated from the Shared Pool.
Database Systems
Slide 23
Java and Streams Pool

Java Pool memory is used in server memory for all session-specific
Java code.

Java Pool memory is used in different ways, depending on what
mode the Oracle server is running in.

In a single database, you can specify that Streams memory be
allocated from a new pool in the SGA called the Streams Pool.

If the size of the Streams Pool is greater than zero, then any SGA
memory used by Streams is allocated from the Streams Pool.

If the size of the Streams Pool is zero, then the memory used by
Streams is allocated from the Shared Pool and may use up to 10% of
the Shared Pool.
Database Systems
Slide 24