OCA 01 - Exploring the Database Architecture

7/29/2019 OCA 01 - Exploring the Database Architecture

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01 - Exploring the Database

Architecture

By Muhammad Asghar Khan

Reference:Apress RMAN Recipes for Oracle Database 11g by Darl Kuhn, Sam Alapati,

and Arup Nanda


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Agenda

http://asghars.blogspot.com

Single-Instance Database Architecture Oracle Instance

Oracle Database

Distributed Systems Architectures

Real Application Clusters

Streams

Data Guard

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Single-Instance Database Architecture


Oracle database consists of two entities: Oracle Instance

A set of processes (Unix) or single threaded process

(Windows) and shared memory area or System Global

Area (SGA)

Oracle Database

A collection of physical OS files or disks in case of ASM

or RAW partitions

Database may be mounted & opened by many

instances, while an instance may mount & open a

single database in its entire lifetime

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Single-Instance Database Architecture(Oracle Instance Memory Structures)


Oracle Instance

System/Shared Global Area

SGA allocates & de-allocates space in units of granules (4MB,8MB or 16MB)

SGA memory is declared by the SGA_TARGET initialization

parameter

SGA contains:

Required

Database Buffer Cache: Buffer Pools etc.

Redo Log Buffer Cache: Redo entries

Shared Pool: Library Cache, Dictionary Cache

Fixed SGA: Bootstrap of SGA

Optional

Large Pool: Shared Server Session, RMAN backup/restore

Java Pool: Java Code

Streams Pool: Queue, Messages

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Database Buffer Cache

The data blocks containing the data of interest are firstcopied into the database buffer cache

Changes (INSERT,UPDATE, DELETE) are applied to copiesof the data blocks in the database buffer cache

When querying data the rows of interest are copies intothe database buffer cache; the relevant rows are thentransferred into the sessions PGA for further processing

Blocks containing data that is frequently accessed will

be in the database buffer cache, therefore minimizingthe need for disk I/O

A well-tuned database buffer cache can result in acache hit ratio well over 90 percent

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The buffer will become dirty when the block in it is updated,

at which point the buffer will be clean again The write to the datafiles is done by the database writer

background process

The size of the database buffer cache is critical forperformance

An undersized cache will result in excessive disk activity An oversized cache is not so bad but can cause problems;

like startup of an instance may be slower or OS is having toswap pages of virtual memory

Determining the optimal size of the database buffer cache is

application specific and a matter of performance tuning Redo Log Buffer Cache

Short-term staging area for change vectors before they arewritten to the redo log on disk

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A change vector is a modification applied to something

Whenever a data block is changed, the change vectors applied tothe block are written out to the redo log, from where they can beextracted and applied to datafile backups if it is ever necessary torestore a datafile

One write of the log buffer to disk may be a batch of many changevectors from many transactions

The writes are done by the log writer background process (LGWR)

Write occurs in real time, and while it is in progress, the sessionthat issued the COMMIT will hang

It is a circular buffer

The commit-complete message is not returned to the session until

the data blocks in the cache have been changed (which meansthat the transaction has been completed) and the change vectorshave been written to the redo log on disk (and therefore thetransaction could be recovered if necessary)

The size of log buffer cache is determined by the Oracle server andis based on the number of CPUs on the server node

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You cannot do DML faster than the LGWR can flush the

change vectors to the online redo log files If redo generation is the limiting factor in a databases

performance, the only option is to go to RAC

Shared Pool Memory in the shared pool is allocated according to an LRU

(least recently used) algorithm It is divided into dozens of substructures, all of which are

managed internally by the Oracle server

Four of the shared pool components are: Library Cache: Stores recently executed code, in its parsed form. In a

well-designed application, it is possible that statements may be parsedonce and executed millions of times

Data Dictionary Cache/Row Cache: Stores recently used objectdefinitions: descriptions of tables, indexes, users, and other metadatadefinitions

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PL/SQL Area: Stored PL/SQL objects are stored in PL/SQL area.

Anonymous PL/SQL cannot be cached

SQL Query and PL/SQL Function Result Cache: The result cache

is a release 11g new feature. A result cache lets the Oracle

server store the results of frequent queries or PL/SQL in

memory. By default, use of the SQL query and PL/SQL functionresult cache is disabled

Fixed SGA

Contains a set of variables that point to other

components of the SGA Acts like a bootstrap section of the SGA

It is fixed for each release of Oracle and cant be altered

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Large Pool

Major use of the large pool is by shared server processes

Large pool may also be used by the Recovery Manager

Java Pool

Only required if your application is going to run Java-storedprocedures within the database

It is used for the heap space needed to instantiate the Java

objects

Note that Java code is not cached in the Java pool: it iscached in the shared pool

Optimal size of the Java pool is dependent on the Java

application

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Streams Pool Used by Oracle Streams

In streams, change vectors from the redo log are used

to reconstruct the statements that were executed

These statements are executed at the remote database Stream pool is used for these operations

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EXERCISE 1-1: Investigate the Memory Structures ofthe Instance

1. Connect as user SYSTEM

1. Show the sizes of the SGA components that can bedynamically resized

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Single-Instance Database Architecture(Oracle Instance - Processes)


Processes User Processes

Application program or Oracle tool creates a user process on

the server

Run the application or Oracle tool code

Server Processes/ Foreground Processes

Services an individual user process

With each server process is associated a private non-shareable

memory, called the program global area (PGA)

Server process can be configured as dedicated or shared server

In dedicated mode; listener spawn a new dedicated server

process for each user process

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In shared mode; listener handover the connection to dispatcher

which connects the user process to the shared server process

Background Processes

An Oracle instance can have many background processes

Each Oracle background process is in charge of a separate task

Some of the BG processes can be:

Database Writer (DBWn)

Log Writer (LGWR)

Checkpoint (CKPT)

System Monitor (SMON)

Process Monitor (PMON)

Manageability Monitor (MMON)

Memory Manager (MMAN)

Archiver (ARCn)

Recoverer (RECO)

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Database Writer (DBWn) DBWn writes dirty buffers from the database buffer cache to the

datafiles

It is possible for an instance to have several database writers (up to amaximum of twenty), which will be called DBW0, DBW1, and so on

The default number is one database writer per eight CPUs

No free buffers, too many dirty buffers, a three-second timeout, or a

checkpoint will cause the DBW to write A free buffer is a buffer that is neither dirty (updated, and not yet

written back to disk) nor pinned (a pinned buffer is one that is beingused by another session at that very moment)

DBW does absolutely nothing when a transaction is committed,however; when a checkpoint occurs, all dirty buffers are written

During a checkpoint, disk I/O rates will hit the roof, CPU usage may goto 100 percent, end user sessions will experience degradedperformance, therefore; it must be avoided

Partial checkpoints occur only when a datafile or tablespace is takenoffline; when a tablespace is put into backup mode; when a tablespaceis made read only

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Log Writer (LGWR) LGWR writes (flushes) the contents of the log buffer to the online log files When a session makes any change to blocks in the database buffer cache,

before it applies the change to the block it writes out the change vectorthat it is about to apply to the log buffer

When a session issues a COMMIT, the LGWR writes in real time: thesession hangs, while LGWR writes the buffer to disk

It is impossible to do DML faster than LGWR can write the change vectorsto disk

LGWR flush the log buffer to disk; on COMMIT, when the buffer is one-third full, just before DBWn writes

As DBWn writes after three-second timeout, therefor, LGWR will alwayswrite before three-second timout

It is necessary to know that it is perfectly possible for DBWn to write anuncommitted transaction to the datafiles, as long as the undo data isavailable

Undo data also generates change vectors: as these will be in the redo logfiles before the datafiles are updated which can be used to roll back atransaction

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Checkpoint (CKPT) Checkpoint instructs DBWn to write out dirty buffers at a constant

rate, so that there is always a predictable gap between DBWn (whichwrites blocks on a lazy algorithm) and LGWR (which writes changevectors in near real time)

This is also called incremental checkpoints

The current checkpoint position, also known as the RBA (the redo byte

address), is the point in the redo stream at which recovery must beginin the event of an instance crash

CKPT continually updates the controlfile with the current checkpointposition

System Monitor (SMON) SMON mounts a database by locating and validating the database

controlfile It then opens a database by locating and validating all the datafiles

and online log files

Once the database is opened and in use, SMON is responsible forvarious housekeeping tasks, such as collating free space in datafiles

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Process Monitor (PMON) If the session (user process) is terminated in a disorderly manner

(perhaps because the users PC is rebooted), then the session will beleft in a state that must be cleared up

If a session has terminated abnormally, PMON will destroy the serverprocess, return its PGA memory to the operating systems freememory pool, and roll back any incomplete transaction that may havebeen in progress

Manageability Monitor (MMON) MMON regularly (by default, every hour) captures statistics

(snapshots) from the SGA and writes them to the data dictionary (keptfor only eight days)

Every time MMON takes a snapshot, it also launches the AutomaticDatabase Diagnostic Monitor (ADDM)

The ADDM is a tool that analyses database activity using an expertsystem developed over many years by many DBAs

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MMON also continuously monitors the database and the instance to

check whether any alerts should be raised Memory Manager (MMAN) It enables the automatic management of memory allocations

The DBA need to set an overall target for memory usage, and MMANwill observe the demand for PGA memory and SGA memory, andallocate memory to sessions and to SGA structures as needed

Archiver (ARCn) The process and purpose of launching ARCn to create archive log files

There can be from one to thirty, named ARC0, ARC1, and so on

In order to preserve a complete history of all changes applied to thedata, the online log files must be copied as they are filled and beforethey are reused, the ARCn is responsible for doing this

Most production transactional databases will run in archive log mode,meaning that ARCn is started automatically and that LGWR is notpermitted to overwrite an online log file until ARCn has successfullyarchived it to an archive log file

If archiving fails, the database will eventually hang

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Recoverer (RECO)

A distributed transaction is a transaction that involves updates

to two or more databases

Distributed transactions are designed by programmers and

operate through database links

Distributed transactions require a two-phase commit

If anything goes wrong anywhere between the two phases,

RECO takes action to cancel the commit and roll back the work

in all databases

EXERCISE 1-2: Investigate the Processes Running inYour Instance

1. Determine what processes are running

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2. Demonstrate the launching of server processes as

sessions are made

1. Count the number of processes running that have the string

oracle in their name; this will include all the session server

processes (and possibly a few others)

1. Launch a SQL*Plus session, and rerun the preceding

command: use the host command to launch an

operating shell from within the SQL*Plus session

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Observe how the number of processes changes from 81

to 82 : the difference is the launching and terminating of

the server process supporting the SQL*Plus session

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Single-Instance Database Architecture(Oracle Database)


Oracle Database

As per the RDBMS standard the Oracle architectureguarantees abstraction of the logical from the physical

The relationship between physical and logical structures ismaintained and documented in the data dictionary ,

which contains metadata describing the whole database Physical Structure

Physical structures are visible to the system administrators

Data Files

Control Files

Online Redo Log Files Archive Log Files

Parameter Files

Trace Files

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Alert Files

Password Files

Logical Structure

Logical structures (segments) are visible to the

programmers Tablespaces

Segments

Extents

Blocks

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Single-Instance Database Architecture(Oracle Database - Physical)


Data Files Datafiles are the repository for data Oracle database requires at least two data files

At the operating system level, a datafile consists of a numberof operating system blocks

Internally, datafiles are formatted into Oracle blocks

The block size is a matter for tuning and can range (withlimits depending on the platform) from 2 KB up to 64 KB

Many DBAs like to match the operating system block size tothe Oracle block size. For performance reasons, theoperating system blocks should never be larger than the

Oracle blocks Within a block, there is a header section and a data area,

and possibly some free space

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The header section contains information such as the row

directory , which lists the location within the data area of

the rows in the block (if the block is being used for a table

segment) and also row locking information if there is a

transaction working on the rows in the block

Server processes read from the datafiles; DBWn writes todatafiles

Control Files

It contains pointers to the rest of the database:

Locations of the online redo log files and of the datafiles, and of themore recent archive log files if the database is in archive log mode

It also stores information required to maintain database integrity:

various critical sequence numbers and timestamps

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Every database has one controlfile, but a good DBA will

always create multiple copies of the controlfile so that if one

copy is damaged, the database itself will survive

All organizations should have a DBA standards handbook,

which will state something like all production databases will

have three copies of the controlfile Online Redo Log Files

It stores a continuous chain in chronological order of every

change vector applied to the database

If a datafile is damaged, these change vectors can be appliedto datafile backups to redo the work, bringing them forward

in time until the moment that the damage occurred

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Every database has at least two online redo log files,

but as with the controlfile, a good DBA creates multiplecopies of each online redo log file

The online redo log consists of groups of online redo logfiles, each file being known as a member

An Oracle database requires at least two groups of atleast one member each to function

The size and number of your log file groups are a matterof tuning

Archive Log Files When an online redo log file fills, the ARCn process

copies it out of the database to an archive redo log file

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Parameter Files

When an Oracle instance is started, the SGA structuresbuild in memory and the background processes startaccording to settings in the parameter file

This is the only file that needs to exist in order to start

an instance There are several hundred parameters, but only one is

required: the DB_NAME parameter

Alert Log and Trace Files

The alert log is a continuous stream of messagesregarding certain critical operations affecting theinstance and the database

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Trace files are generated by background processes

when they detect error conditions, and sometimes to

report certain actions

Password Files

In this file you specify the names of database users whohave been granted the SYSDBA or SYSOPER privilages

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Single-Instance Database Architecture(Oracle Database-Logical)


Tablespaces

Oracle abstracts the logical storage from the physical storage by means ofthe tablespace

A tablespace is logically a collection of one or more segments, andphysically a collection of one or more datafiles

Each database has five default tablespaces: system, sysaux, undo,temporary and default tablespace

Segments Oracle uses the term segment to describe any structure that contains

data like table

Segment can be of type data, index or temporary

Extents

An extent is a series of blocks that are consecutively numbered within adatafile, and segments will grow by adding new extents to them

These extents need not be adjacent to each other, or even in the samedatafile; they can come from any datafile that is part of the tablespacewithin which the segment resides

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Single-Instance Database Architecture(Oracle Database-Logical)


Blocks

Oracle block is the smallest unit of space allocation in

Oracle

Internally, datafiles are formatted into Oracle blocks

The block size is a matter for tuning and can range (withlimits depending on the platform) from 2 KB up to 64

KB

Many DBAs like to match the operating system block

size to the Oracle block size. For performance reasons,the operating system blocks should never be larger than

the Oracle blocks

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EXERCISE 1-3: Investigate the Storage Structures in

Your Database

1. Create a table without nominating a tablespace

2. Identify the tablespace in which the table resides,the size of the extent, the file number the extent is

in, and which block of the file the extent starts at

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3. Identify the file by name

4. Work out precisely where in the file the extent is, interms of how many bytes into the file it begins

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In a distributed environment, there are variouspossibilities for grouping instances and databases:

Real Application Clusters (RAC), where multipleinstances open one database

Streams, where multiple Oracle servers propagatetransactions between each other

Data Guard, where a primary database updates astandby database

Real Application Clusters RAC database can be configured for 100 percent

uptime

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Sessions against the failed instance can bereestablished against a surviving instance without theend user being aware of any disruption

Streams

There are various circumstances that make itdesirable to transfer data from one database toanother

Streams can also be used for fault tolerance

Data Guard A physical standby is byte-for-byte identical with the

primary, for the purpose of zero data loss

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A logical standby contains the same data as the

primary, but possibly with different data structures

EXERCISE 1-4: Database Is Single Instance or Part of

a Distributed System

1. Determine if the instance is part of an RAC database

2. Determine if the database is protected against dataloss by a standby database

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htt // h bl t

3. Determine if Streams has been configured in the

database

This return no rows, which shows Streams has neverbeen configured

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OCA 01 - Exploring the Database Architecture

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