The document provides a comprehensive overview of Oracle architecture, covering key components such as databases, instances, memory areas, and various processes involved in data management and transaction processing. It details the roles of different processes like the database writer (DBWR), log writer (LGWR), and archiver (ARCH), along with descriptions of memory structures like the System Global Area (SGA) and Program Global Area (PGA). Additionally, it explains the types of files used in Oracle, including data, log, control, and initialization files, and highlights critical functionalities such as SQL execution and memory management.

1. Oracle Architecture

2. Oracle Architecture
Database : An organized collection of data stored in
data files
Instance : Set of processes and memory areas that
provide the capability of accessing the data
RDBMS : A database satisfying most of the Codd’s rule

3. Oracle Architecture
Database
Instance 1
(SGA + Processes)
Instance 2
(SGA + Processes)
Instance and Databases

4. Pieces of the Oracle RDBMS
ORACLE RDBMS
Others
Applications
Query Tools
Network
Interfaces
Memory Areas
Processes
Disk Data Storage

5. Parts of Oracle RDBMS
Background Processes : To perform tasks like data writing, log
writing, archiving etc.
Shared Memory Areas : To keep the things that can be shared
across users of an instance
Disk Data Storage : To actually store the data

6. The Oracle Processes
A simplified view
End
Users
User
Service
Processes
Memory
Areas
Oracle
Monitors
Log
writers
Data
writers
Log
Files
Data
Files

7. The Oracle Processes
User Service Process : Your Liaisons to Oracle
Data Writers : Writes Data to the data files
from memory areas
Log Writers :Record each transaction in a
separate file from the data files
Oracle Monitors :Monitors the functioning of the
database

8. Memory Areas
A simplified view
Users
Background Processes
Software
Code
Areas
System
Global
Area
Program
Global
Area

9. Memory Areas
Software Code Areas : Home to the software containing all
the functionality from Oracle
database
System Global Area : Heart of an Oracle instance. Shared area
for all users in an instance
Program Global Area : Information used by a single user
process

10. Disk Storage
Basic Types of Files
Initialization
Files
SGA
Definition
Files
Control
Files
Log &
Error
Files
Data Files
Online
Redo
Log
Files
Archive
Logs
Start Up
Oracle
Processes

11. Disk Storage
Data Files:
• Records of data arranged in a database
• Other database objects like indexes, views, packages etc.

12. Disk Storage
Online redo Log Files
• Record of every transaction made to the database
• Used for recovery
Archive Log Files
• Background process writes the copy of the redo log file to
this file
• On a magnetic disk drive or a magnetic tape

13. Disk Storage
Initialization Files
• Equivalent of autoexec.bat and config.sys
• Inittest.ora and configtest.ora : Contains
• Values of tunable parameters
• Locations of control files and archive log files
• Locations for some of the log and error files

14. Disk Storage
Control Files
• Where the datafiles are
• Number of latest transaction to the database
• Binary format

15. Disk Storage
SGA Definition File
• Some details about creating the SGA on startup
Log and error Files
• All major database activities in log files
• When oracle monitoring processes detect a problem
they log it into a series of error files

16. Oracle Memory Structures
• Mostly real memory and not virtual memory
• Permission on the operating system to setup shared
memory areas is automatically done
• SGA, PGA, Software code areas

17. SGA (System Global Area)
Database
Buffer
Cache
Redo
Log
Buffer
Shared
Pool
Multi-
Threaded
Server
Queues
Inter-Process
Communication
Area
Shared
SQL
Areas
Data
Dictionary
Cache
Cursors
SQL Oracle DB
Schema
Recursive
Calls and
PL/SQL
Datafiles Redo
Log
Files

18. SGA
Database Buffer Cache
• All reads and writes are in blocks
• Rows read from datafiles or rows that need to be
written to datafiles
• How long do the rows remain there?
• New or modified records:Written by the Database
Writer process
• Read but not modified rows:Least Recently Used
Algorithm (LRU)

19. SGA
Redo Log Buffer
• Way-station between oracle-process that creates a
database update and redo log files
• First in First out approach
• Log writer writes these buffers to redo log files

20. SGA
Shared Pool
Shared SQL Areas (Library Cache):
• Parsed query for any uses to access
• Modified form of LRU
Data Dictionary Cache (Row Cache):
• Info about tables, there location, access privileges etc.
Cursors
• Areas in memory for further processing of retrieved data
• Implicit cursors, recursive calls (create table) and explicit
PL/SQL cursors
• PGA contains pointers to SGA for cursors

21. Program Global Area (PGA)
• Owned by a single-user process
• If no memory is available on your computer, you receive
an oracle error to that effect
• Allocated on the machine that acts as a database server
• Also known as Process Global Area
• Size depends on Oracle version running on the operating
system.
• Parameters affecting size :
• Open database links #
• Database files #
• Log Files #

22. User Work Spaces
• Memory areas for applications to run
Software
Code
Areas
for Oracle
Processes
Software
Code
Areas
for User
Processes
Sort Areas

23. User Work Spaces
Software Code Area for Oracle Processes:
• Owned by Oracle User, protected by operating system,
sharable
• Desirable to have one copy running on multiple instances
Software Code Area for User Processes:
• For custom applications
• Area required is much smaller than SGA
• Inactive users can be swapped out to disk (virtual memory)

24. User Work Spaces
Sort Area
• For sorting
• Maximum size : SORT_AREA_SIZE
• Minimum size : SORT_AREA_RETAINED_SIZE (reduced to
this parameter after sorting is over)
• If data to sort does not fit in memory ,data is sorted in
chunks and chunks are sorted

25. Oracle Files in Detail
Location (Optimal Flexible Architecture For Unix)
Tar admin data local product
Oracle Base
7.3.1 7.3.2
bin dbs sqlplus rdbms
ORACLE HOME
adhoc arch bdump cdump exp logbook pfile temp udump

26. Oracle Files in Detail
What is where at a broad level
TAR :Tape downloads
local : Items you create and may want to use with Oracle software
admin : log files and other information for DBA
data :datafiles
product :Oracle Application software and configuration files for the
Oracle home instances
• Datafiles storage is altered in large applications for performance.

27. Oracle Files
Data Files
Tablespace
users
Tablespace
System
Database
Physical View
Logical View
Tablespace users
Tablespace system
Table Index
Table Index
Table
Data
file
Data
file
Data
file
Data
file

28. Oracle Files
Data Files
• Balance the load in tablespaces in different datafiles
•Number of files down to a reasonable value
• Contains latest system change number(helps in recovery)
• Pre-allocated to the size given in the creation command
• Raw and cooked datafiles
• Raw datafiles : Oracle software talks directly to disk
drives(raw disks)

29. Oracle Files
Data Files
• Having number of tablespaces across several disks improves
performance
• Should be on a separate disk drive from those of redo log files
• Read only tablespaces : No need to check SCN. Can be placed
on CD-ROMs
• Info about datafiles can be queried from sys.dba_data_files

30. Oracle Files
Redo Log Files
• Have a small number of large redo log files (not too fast
log switches)
• Used in a cyclical manner
• You can not overwrite the contents of a redo log file until
it has been completely copied to an archive log file in
archive log mode
• Groups of redo log files
• Info about these files can be queried from v$logfile

31. Oracle Files
Archiving to Log Files
• Disk or magnetic tape
• What u need to do ?
• Set LOG_ARCHIVE_START parameter to TRUE in
init.ora
• Tell it the format for the name of your archive log file
names :
LOG_ARCHIVE_DEST = /disk32/oracle/logs/alog
LOG_ARCHIVE_FORMAT = ‘%s.arc’
• Shutdown the instance and perform a full backup

32. Oracle Files
Archiving to Log Files
• What u need to do ?
• Bring up the instance with database not open
(startup nomount)
• alter database archivelog;
• Open the database .Verify that archive logs are
actually created.Force a redo log switch with the
command
alter system switch log file.

33. Oracle Files
Control Files
• Can not be edited
• Contains info about name of db, datafiles, redo log files,
and a record of when they were created
• Have at least two control files. They will be identical
• When one is corrupted, copy the other one and restart
the system
• Spread control files over separate disks

34. Oracle Files
Initialization Files
• init.ora (same for more than one instance)
ORACLE_HOME/dbs/ initmarscash.ora.
• config.ora (specific to an instance)
ORACLE_BASE/admin/instance_name/pfile/ configmarscash.ora
In init.ora set ifile=filename with path of config.ora

35. Oracle Files
Log Files
• alert_marscash.ora
• Contains :Major DBA activities that occur
Internal Oracle errors
Messages related to multi-threaded server
Errors related to snapshots refresh

36. Oracle Files
Trace files
• Parameter in initialization files :BACKGROUND_DUMP_DEST
• Typically ORACLE_BASE/admin/instance_name/bdump
• Produced by Oracle background processes
• Contains : Date and time of the problem
Process with which the problem occurred
Numbered Oracle error messages.
Clean up your log and trace files and keep a backup

37. Oracle Processes
Database writer processes (DBWR)
• Can have more than one DBWR (DB_WRITERS in init.ora)
• Writes dirty blocks to disks
• One of the enforcers of LRU. Chooses records that are inactive.
• Called into action when :
•No of dirt buffers > ½ * DB_BLOCK_WRITE_BATCH
• A server process does not find a free buffer after scanning
DB_BLOCK_MAX_SCAN_CNT buffers
• DBWR wakes up routinely after 3 seconds
• When a checkpoint occurs

38. Oracle Processes
Database writer processes (DBWR)
When How much
• No of dirty buffers > Scans a list of dirty buffers
½ * DB_BLOCK_WRITE_BATCH starting at LRU end and
writes in batch
• A server process does not find a Same as above
free buffer after scanning
DB_BLOCK_MAX_SCAN_CNT buffers
• DBWR wakes up routinely after 3 searches
seconds 2*DB_BLOCK_WRITE_BATCH
• Checkpoint occurs All checkpoint records

39. Oracle Processes
Checkpoint (CKPT)
• Updates the datafile headers in the datafiles and control
files to record the SCN
• Occurs when :
•An online redo log file fills
• LOG_CHECKPOINT_INTERVAL in init.ora
• CHECKPOINT_PROCESS = TRUE in init.ora
• Only if high transaction volume delaying log switches
• Also done by the Log Writer(LGWR) in its free time

40. Oracle Processes
Log Writer (LGWR)
• Writes from redo log buffer to redo log files on first in first
out basis in a cyclical manner
• When activated
• Commit from a user process
• Every 3 seconds
• Redo log buffer is 1/3rd full
• When the DBWR writes to datafiles, LGWR writes the
corresponding records to redo log files
•Updates the datafile headers in the DB buffer and control
files to record the SCN

41. Oracle Processes
Archiver (ARCH)
• Writes copy of a redo log file to archive log file
• When an online redo log file is full
• May cause contentions
Recoverer (RECO)
• Problems with distributed transaction
• Waits until connection is re-established ,goes over doubtful
transactions and resolves any updates on the remote system

42. Oracle Processes
Lock Writer(LCKn)
• For parallel server (Multiple instances on different nodes
sharing the same physical database)
• LCK0 to LCK9 depending on GC_LCK_PROCS in init.ora
• They talk to one another and ensure that records are
properly locked
Dedicated Server Process
• Interface between user processes and memory areas and
disks
• All user get this process and it remains until they
disconnect

43. Oracle Processes
System Monitor(SMON)
• Cleanup after the Oracle instance
• Instance recovery at start up
• Cleanup instances periodically
• Coalesces free space
• Called by other processes to do some free up say temporary
segment

44. Oracle Processes
Process Monitor(PMON)
• Cleans up after failed user processes
• Wakes up periodically
• Also called by other processes
• In MTS environment restarts the dispatcher or server
processes that have died (Only if the no < minimum in
inti.ora)

45. Oracle Architecture
An integrated simplified view
DB
Buffer
Cache
Server
Process
DBWR CKPT LGWR ARCH
SMON
Datafiles Control
Files
Redo
logs
Archived
Logs
PMON
PGA
Redo
Log
Buffers
Memory
Processes
Files
User

46. An Integrated View
Library
Cache
Data
Dictionary
Cache
UGA IO Buffer Area
Shared SQL Area
Database Buffer Cache
Large Pool
Redo Log Buffer
System Global Area
Instance Lock area
PMON
SMON
Snnnn
Dnnnn
Pnnnn
SNPnn
RECO
LCKn
GMS
LMON
LMDO
IOrnn TRWR
SYSTEM
ROLLBACK
TEMP
INDEX
USER
DATA
Datafiles
Control Files
Archive
Logs
Alert File
Trace File
Redo Logs
Group 1
Group 2
DATABASE
INSTANCE
SERVER
Processes
PGA
Parameter
File
Password
File
User
DBWR CKPT LGWR ARCH
INIT.ORA
orapwSID

47. The Logical Structure
Database Tablespace Datafiles
Owners
Schemas Segments Extents

48. The Logical Structure
Fixed Block Header
Variable Block Header
Row Header Row data
Row Header Row data
Row Header Row data
PCTFREE
Database Block

49. Database Block
Row Data
• Table or index data
• Rows can span blocks
Row Header
• Row Number and column Names
Free Space
• To insert new rows and for updates to rows
• PCTFREE and PCTUSED

51. ROWID
Oracle 7
BBBBBBBB. RRRR. FFFF
Block Number Row Number File Number
Oracle 8
Base 16
BBBBBBBB
OOOOOO. FFFF
Block Number
Object Number File Number
Base 64
SSS.
Slot Number

52. SQL Execution
1. Parse - Compile SQL statement in shared pool and check
user privileges
2. Execute - Parsed code is executed
3. Fetch - Data retrieved and returned to the user

53. SQL Execution in detail
1) OPEN SQL Statement
• Obtain cursor(memory address) in library cache of
shared pool
• Obtain private sql area in PGA where statement
return values will be stored
2) PARSE SQL Statement
• Check syntax
• Check objects referred and permissions
• Create execution plan (also called a parse tree) and
put into the shared pool

54. SQL Execution in detail
3) Create BIND variables (only for select)
4) EXECUTE the statement.
• Applies parse tree to data buffers
• Performs logical/physical reads
• Performs constraint checking
• Changes data if needed.
5) FETCH (Only for select)
Returns dataset into BIND variables

55. SQL Execution in detail
Select
• If Blocks is in the memory, perform logical read.
• If not in memory, perform physical read
• If index exist server process reads index blocks, actual data
blocks are read into database buffer using rowid
• If no index, full table scan performed, every block are read
into buffer cache until required row found

56. SQL Execution in detail
Update
• Acquire data blocks into the db buffer cache
• Acquire rollback blocks into the database buffer cache
• Place exclusive row locks on rows that are about to change .
• Store record in the redo log buffer
• Save rollback data into a rollback segment block buffer
• Applies changes to the database block buffer

57. SQL Execution in detail
Fast Commit
1. The server puts the commit record and SCN into the redo
log buffer.
2. LGWR does contiguous write of all redo log buffer data up to
and including the commit record to the redo log file.
3. User is notified of commit completion.
4. Server process indicates to Oracle that the transaction is
complete and that locks can be released.

58. References
 Oracle DBA Survival Guide - Greene
 Oracle DBA Handbook – Kevin Loney
 http://metalink.oracle.com