Oracle Architectural Components
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Objectives
After completing this lesson, you should be able to do
the following:
• Outline the Oracle architecture and its main
components
• List the structures involved in connecting a user to
an Oracle Instance
Overview of Primary Components
The Oracle architecture includes a number of primary components, which are discussed further in this lesson.
Oracle server: There are several files, processes, and memory structures in an Oracle server; however, not all of them are used when processing a SQL statement. Some are used to improve the performance of the database, ensure that the database can be recovered in the event of a software or hardware error, or perform other tasks necessary to maintain the database. The Oracle server consists of an Oracle Instance and an Oracle database.
Oracle instance: An Oracle Instance is the combination of the background processes and memory structures. The instance must be started to access the data in the database. Every time an instance is started, a System Global Area (SGA) is allocated and Oracle background processes are started. Background processes perform functions on behalf of the invoking process. They consolidate functions that would otherwise be handled by multiple Oracle programs running for each user. The background processes perform input/output (I/O) and monitor other Oracle processes to provide increased parallelism for better performance and reliability.
Oracle database: An Oracle database consists of operating system files, also known as database files, that provide the actual physical storage for database information. The database files are used to ensure that the data is kept consistent and can be recovered in the event of a failure of the instance.
Other key files: Nondatabase files are used to configure the instance, authenticate privileged users, and recover the database in the event of a disk failure.
User and server processes: The user and server processes are the primary processes involved when a SQL statement is executed; however, other processes may help the server complete the processing of the SQL statement.
Other processes: Many other processes exist that are used by other options, such as Advanced Queuing, Real Application Clusters, Shared Server, Advanced Replication, and so on. These processes are discussed within their respective courses.
Oracle Server
An Oracle server:
Is a database management system that provides an open, comprehensive, integrated approach to information management
Consists of an Oracle Instance and an Oracle database
Oracle Server
The database server is the key to information management. In general, a server must reliably manage a large amount of data in a multiuser environment so that many users can concurrently access the same data. All this must be accomplished while delivering high performance. A database server must also prevent unauthorized access and provide efficient solutions for failure recovery.
Oracle Instance
An Oracle Instance:
Is a means to access an Oracle database
Always opens one and only one database
Consists of memory and background process structures
Oracle Instance
An Oracle Instance consists of the System Global Area (SGA) memory structure and the background processes used to manage a database. An instance is identified by using methods specific to each operating system. The instance can open and use only one database at a time.
Establishing a Connection and Creating a Session
Connecting to an Oracle Instance:
Establishing a user connection
Creating a session
Establishing a Connection and Creating a Session
Before users can submit SQL statements to an Oracle database, they must connect to an instance.
The user starts a tool such as SQL*Plus or runs an application developed using a tool such as Oracle Forms. This application or tool is executed as a user process.
In the most basic configuration, when a user logs on to the Oracle server, a process is created on the computer running the Oracle server. This process is called a server process. The server process communicates with the Oracle Instance on behalf of the user process that runs on the client. The server process executes SQL statements on behalf of the user.
Connection:
A connection is a communication pathway between a user process and an Oracle server. A database user can connect to an Oracle server in one of three ways:
The user logs on to the operating system running the Oracle Instance and starts an application or tool that accesses the database on that system. The communication pathway is established using the interprocess communication mechanisms available on the host operating system.
The user starts the application or tool on a local computer and connects over a network to the computer running the Oracle Instance. In this configuration, called client-server, network software is used to communicate between the user and the Oracle server.
In a three-tiered connection, the user’s computer communicates over the network to an application or a network server, which is connected through a network to the machine running the Oracle Instance. For example, the user runs a browser on a computer on a network to use an application residing on an NT server that retrieves data from an Oracle database running on a UNIX host.
Sessions
A session is a specific connection of a user to an Oracle server. The session starts when the user is validated by the Oracle server, and it ends when the user logs out or when there is an abnormal termination. For a given database user, many concurrent sessions are possible if the user logs on from many tools, applications, or terminals at the same time. Except for some specialized database administration tools, starting a database session requires that the Oracle server be available for use.
Note: The type of connection explained here, where there is a one-to-one correspondence between a user and server process, is called a Dedicated Server connection. When using a Shared Server configuration, it is possible for multiple user processes to share server processes.
Oracle Database
An Oracle database:
Is a collection of data that is treated as a unit
Consists of three file types
Oracle Database
The general purpose of a database is to store and retrieve related information. An Oracle database has a logical and a physical structure. The physical structure of the database is the set of operating system files in the database. An Oracle database consists of three file types.
Datafiles containing the actual data in the database
Redo log files containing a record of changes made to the database to enable recovery of the data in case of failures
Control files containing information necessary to maintain and verify database integrity
Other key file structures
The Oracle server also uses other files that are not part of the database:
The parameter file defines the characteristics of an Oracle Instance. For example, it contains parameters that size some of the memory structures in the SGA.
The password file authenticates users privileged to start up and shut down an Oracle Instance.
Archived redo log files are offline copies of the redo log files that may be necessary to recover from media failures.
Physical Structure
The physical structure includes three types of files:
Control files
Datafiles
Redo log files
Physical Structure
The physical structure of an Oracle database includes three types of files: control files, datafiles, and redo log files.
Memory Structure
Oracle’s memory structure consists of two memory areas known as:
System Global Area (SGA): Allocated at instance startup, and is a fundamental component of an Oracle Instance
Program Global Area (PGA): Allocated when the server process is started
System Global Area
The SGA consists of several memory structures:
-Shared Pool
-Database Buffer Cache
-Redo Log Buffer
-Other structures (for example, lock and latch management, statistical data)
There are two additional memory structures that can be configured within the SGA:
-Large Pool
-Java Pool
System Global Area (SGA)
The SGA is also called the shared global area. It is used to store database information that is shared by database processes. It contains data and control information for the Oracle server and is allocated in the virtual memory of the computer where Oracle resides.
The following statement can be used to view SGA memory allocations:
SQL> SHOW SGA:
Total System Global Area 36437964 bytes
Fixed Size 6543794 bytes
Variable Size 19521536 bytes
Database Buffers 16777216 bytes
Redo Buffers 73728 bytes
Dynamic SGA:
Beginning with Oracle9i, a dynamic SGA implements an infrastructure that allows the SGA configuration to change without shutting down the instance. This then allows the sizes of the Database Buffer Cache and Shared Pool to be changed without shutting down the instance. Conceivably, the Database Buffer Cache and Shared Pool could be initially under-configured and would grow and shrink depending upon their respective work loads, up to a maximum of SGA_MAX_SIZE.
Sizing the SGA:
The size of the SGA is determined by several initialization parameters. The parameters that most affect SGA size are:
DB_CACHE_SIZE: The size of the cache of standard blocks. The default is 48 MB on UNIX and 52 MB on NT
LOG_BUFFER: The number of bytes allocated for the Redo Log Buffer
SHARED_POOL_SIZE: The size in bytes of the area devoted to shared SQL and PL/SQL. The default is 16 MB. If 64 bit, then the default size is 64 MB
LARGE_POOL_SIZE: The size of the Large Pool. The default is zero (Unless the init.ora parameter PARALLEL_AUTOMATIC_TUNING is set to TRUE, then the default is automatically calculated.)
JAVA_POOL_SIZE: The size of the Java Pool. The default is 24 MB
Therefore, the SGAs size can not exceed SGA_MAX_SIZE - DB_CACHE_SIZE - LOG_BUFFER – SHARED_POOL_SIZE - LARGE_POOL_SIZE - JAVA_POOL_SIZE.
Note: Dynamic SGA and sizing are covered in further detail in the Oracle9i Database Performance Tuning course.
SGA is dynamic
Sized by the SGA_MAX_SIZE parameter
Allocated and tracked in granules by SGA components
Contiguous virtual memory allocation
Granule size based on total estimated SGA_MAX_SIZE
Unit of Allocation:
A granule is a unit of contiguous virtual memory allocation. The size of a granule depends on the estimated total SGA size whose calculation is based on the value of the parameter SGA_MAX_SIZE.
4 MB if estimated SGA size is < 128 MB
16 MB otherwise
The components (Database Buffer Cache and Shared Pool) are allowed to grow and shrink based on granule boundaries. For each component which owns granules, the number of granules allocated to the component, any pending operations against the component (for example, allocation of granules through ALTER SYSTEM, freeing of granules through ALTER SYSTEM, corresponding self-tuning), and target size in granules will be tracked and displayed by the V$BUFFER_POOL view. At instance startup, the Oracle server allocates granule entries, one for each granule to support SGA_MAX_SIZE bytes of address space. As startup continues, each component acquires as many granules as it requires. The minimum SGA configuration is three granules (one granule for fixed SGA [includes redo buffers]; one granule for Database Buffer Cache; one granule for Shared Pool).
Shared Pool
Used to store:
Most recently executed SQL statements
Most recently used data definitions
It consists of two key performance-related memory structures:
Library Cache
Data Dictionary Cache
Sized by the parameter
SHARED_POOL_SIZE
Shared Pool
The Shared Pool environment contains both fixed and variable structures. The fixed structures remain relatively the same size, whereas the variable structures grow and shrink based on user and program requirements. The actual sizing for the fixed and variable structures is based on an initialization parameter and the work of an Oracle internal algorithm.
Sizing the Shared Pool:
Since the Shared Pool is used for objects that can be shared globally, such as reusable SQL execution plans; PL/SQL packages, procedures, and functions; and cursor information, it must be sized to accommodate the needs of both the fixed and variable areas. Memory allocation for the Shared Pool is determined by the SHARED_POOL_SIZE initialization parameter. It can be dynamically resized using ALTER SYSTEM SET. After performance analysis, this can be adjusted but the total SGA size cannot exceed SGA_MAX_SIZE.
Library Cache
Stores information about the most recently used SQL and PL/SQL statements
Enables the sharing of commonly used statements
Is managed by a least recently used (LRU) algorithm
Consists of two structures:
Shared SQL area
Shared PL/SQL area
Size determined by the Shared Pool sizing
Library Cache
The Library Cache size is based on the sizing defined for the Shared Pool. Memory is allocated when a statement is parsed or a program unit is called. If the size of the Shared Pool is too small, statements are continually reloaded into the Library Cache, which affects performance. The Library Cache is managed by an LRU algorithm. As the cache fills, less recently used execution paths and parse trees are removed from the Library Cache to make room for the new entries. If the SQL or PL/SQL statements are not reused, they eventually are aged out.
The Library Cache consists of two structures:
Shared SQL: The Shared SQL stores and shares the execution plan and parse tree for SQL statements run against the database. The second time that an identical SQL statement is run, it is able to take advantage of the parse information available in the shared SQL to expedite its execution. To ensure that SQL statements use a shared SQL area whenever possible, the text, schema, and bind variables must be exactly the same.
Shared PL/SQL: The Shared PL/SQL area stores and shares the most recently executed PL/SQL statements. Parsed and compiled program units and procedures (functions, packages, and triggers) are stored in this area.
Data Dictionary Cache
A collection of the most recently used definitions in the database
Includes information about database files, tables, indexes, columns, users, privileges, and other database objects
During the parse phase, the server process looks at the data dictionary for information to resolve object names and validate access
Caching data dictionary information into memory improves response time on queries and DML
Size determined by the Shared Pool sizing
Data Dictionary Cache
The Data Dictionary Cache is also referred to as the dictionary cache or row cache. This multiple caching of data dictionary information both into the Database Buffer Cache and Shared Pool memory improves performance. Information about the database (user account data, data file names, segment names, extent locations, table descriptions, and user privileges) is stored in the data dictionary tables. When this information is needed by the server, the data dictionary tables are read, and the data that is returned is stored in the Data Dictionary Cache.
Sizing the data dictionary:
The overall size is dependent on the size of the Shared Pool size and is managed internally by the database. If the Data Dictionary Cache is too small, then the database has to query the data dictionary tables repeatedly for information needed by the server. These queries are called recursive calls and are slower than the direct queries on the Data Dictionary Cache as direct queries do not use SQL.
Database Buffer Cache
Stores copies of data blocks that have been retrieved from the datafiles
Enables great performance gains when you obtain and update data
Managed through an LRU algorithm
DB_BLOCK_SIZE determines primary block size
Database Buffer Cache
When a query is processed, the Oracle server process looks in the Database Buffer Cache for any blocks it needs. If the block is not found in the Database Buffer Cache, the server process reads the block from the datafile and places a copy in the Database Buffer Cache. Because subsequent requests for the same block may find the block in memory, the requests may not require physical reads. The Oracle server uses an LRU algorithm to age out buffers that have not been accessed recently to make room for new blocks in the Database Buffer Cache.
Consists of independent sub-caches:
DB_CACHE_SIZE
DB_KEEP_CACHE_SIZE
DB_RECYCLE_CACHE_SIZE
Can be dynamically resized
ALTER SYSTEM SET DB_CACHE_SIZE = 96M;
DB_CACHE_ADVICE set to gather statistics for predicting different cache size behavior
Statistics displayed by V$DB_CACHE_ADVICE
Database Buffer Cache
Sizing the Database Buffer Cache:
The size of each buffer in the Database Buffer Cache is equal to the size of an Oracle block, and it is specified by the DB_BLOCK_SIZE parameter. The Database Buffer Cache consists of independent subcaches for buffer pools and for multiple block sizes. The parameter DB_BLOCK_SIZE determines the primary block size, which is used for the SYSTEM tablespace.
Three parameters define the sizes of the Database Buffer Caches:
DB_CACHE_SIZE: Sizes the default buffer cache only, it always exists and cannot be set to zero
DB_KEEP_CACHE_SIZE: Sizes the keep buffer cache, which is used to retain blocks in memory that are likely to be reused
DB_RECYCLE_CACHE_SIZE: Sizes the recycle buffer cache, which is used to eliminate blocks from memory that have little chance of being reused
Buffer Cache Advisory:
The Buffer Cache Advisory feature enables and disables statistics gathering for predicting behavior with different cache sizes. The information provided by these statistics can help you size the Database Buffer Cache optimally for a given workload. The Buffer Cache Advisory information is collected and displayed through the V$DB_CACHE_ADVICE view.
The Buffer Cache Advisory is enabled via the initialization parameter DB_CACHE_ADVICE. It is a dynamic parameter, and can be altered using ALTER SYSTEM. Three values (OFF, ON, READY) are available.
DB_CACHE_ADVICE parameter values:
OFF: Advisory is turned off and the memory for the advisory is not allocated
ON: Advisory is turned on and both cpu and memory overhead is incurred
Attempting to set the parameter to this state when it is in the OFF state may lead to the following error: ORA-4031 Inability to allocate from the Shared Pool when the parameter is switched to ON. If the parameter is in a READY state it can be set to ON without error since the memory is already allocated.
READY: Advisory is turned off but the memory for the advisory remains allocated. Allocating the memory before the advisory is actually turned on will avoid the risk of ORA-4031. If the parameter is switched to this state from OFF, it is possible than an ORA-4031 will be raised.
Redo Log Buffer
Records all changes made to the database data blocks
Primary purpose is recovery
Changes recorded within are called redo entries
Redo entries contain information to reconstruct
or redo changes
Size defined by LOG_BUFFER
Redo Log Buffer
The Redo Log Buffer is a circular buffer that contains changes made to datafile blocks. This information is stored in redo entries. Redo entries contain the information necessary to recreate the data prior to the change made by INSERT, UPDATE, DELETE, CREATE, ALTER, or DROP operations.
Sizing the Redo Log Buffer:
The size of the Redo Log Buffer is defined by the initialization parameter LOG_BUFFER.
Note: Sizing the Redo Log Buffer is covered in further detail in the Oracle9i Database Performance Tuning course. Refer to the “Managing Redo Log Files” lesson for details regarding redo log files.
Large Pool
An optional area of memory in the SGA
Relieves the burden placed on the Shared Pool
Used for:
Session memory (UGA) for the Shared Server
I/O server processes
Backup and restore operations or RMAN
Parallel execution message buffers
PARALLEL_AUTOMATIC_TUNING set to TRUE
Does not use an LRU list
Sized by LARGE_POOL_SIZE
Large Pool
By allocating session memory from the Large Pool for Shared Server, Oracle XA, or parallel query buffers, Oracle can use the Shared Pool primarily for caching Shared SQL statements. Thus relieving the burden on areas within the Shared Pool. The Shared Pool does not have to give up memory for caching SQL parse trees in favor of Shared Server session information, I/O, and backup and recover processes. The performance gain is from the reduction of overhead from increasing and shrinkage of the shared SQL cache.
Backup and restore:
Recovery Manager (RMAN) uses the Large Pool when the BACKUP_DISK_IO= n and BACKUP_TAPE_IO_SLAVE = TRUE parameters are set. If the Large Pool is configured but is not large enough, the allocation of memory from the Large Pool fails. RMAN writes an error message to the alert log file and does not use I/O slaves for backup or restore.
Parallel execution:
The Large Pool is used if PARALLEL_AUTOMATIC_TUNING is set to TRUE, otherwise, these buffers are allocated to the Shared Pool.
Sizing the Large Pool:
The Large Pool is sized in bytes defined by the LARGE_POOL_SIZE parameter. This parameter is not dynamic.
Large Pool and LRU lists:
The Large Pool does not have an LRU list. It is different from reserved space in the Shared Pool, which uses an LRU list.
Java Pool
Services parsing requirements for Java commands
Required if installing and using Java
Sized by JAVA_POOL_SIZE parameter
Java Pool
The Java Pool is an optional setting but is required if installing and using Java. Its size is set, in bytes, using the JAVA_POOL_SIZE parameter. In Oracle9i, the default size of the Java Pool is 24 MB.
Program Global Area
Memory reserved for each user process connecting to an Oracle database
Allocated when a process is created
Deallocated when the process is terminated
Used by only one process
Program Global Area (PGA)
The Program Global Area or Process Global Area (PGA) is a memory region that contains data and control information for a single server process or a single background process. The PGA is allocated when a process is created and deallocated when the process is terminated. In contrast to the SGA, which is shared by several processes, the PGA is an area that is used by only one process.
Contents of PGA:
The contents of the PGA memory varies, depending whether the instance is running in a Dedicated Server or Shared Server configuration. Generally the PGA memory includes these components:
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 divided into two areas:
Persistent area: Contains bind information, and is freed only when the cursor is closed
Run-time area: Created as the first step of an execute request. For INSERT, UPDATE, and DELETE commands, this area is freed after the statement has been executed. For queries, this area is freed only after all rows are fetched or the query is canceled.
Private SQL area (continued): The location of private SQL area depends on the type of connection established for the session. In a Dedicated Server environment, the private SQL areas are located in the PGA of their server process. In a Shared Server environment, the private SQL areas are located in the SGA.
The management of private SQL areas is the responsibility of the User Process. The number of Private SQL areas that a User Process can allocate is always limited by the initialization parameter OPEN_CURSORS. The default value of this parameter is 50.
Session Memory: Consists of memory allocated to hold a session’s variables and other information related to the session. For a Shared Server environment, the session memory is shared and not private.
SQL Work Areas: Used for memory-intensive operations such as: Sort, Hash-join, Bitmap merge, Bitmap create. The size of a work area can be controlled and tuned.
Beginning with Oracle9i, the size of the work area can be automatically and globally managed. This is enabled by setting the WORKAREA_SIZE_POLICY parameter to AUTO, which is the default, and the PGA_AGGREGATE_TARGET initialization parameter. The PGA_AGGREGATE_TARGET parameter is set by the DBA to specify the target aggregate amount of PGA memory available to the instance. This parameter is only a target and can be dynamically modified at the instance level by the DBA. It will accept a number of bytes, kilobytes, megabytes or gigabytes. When these parameters are set, sizing of work areas becomes automatic and all *_AREA_SIZE parameters are ignored for these sessions.
Prior to Oracle9i, the DBA controlled the maximum size of SQL work areas by setting the following parameters: SORT_AREA_SIZE, HASH_AREA_SIZE, BITMAP_MERGE_AREA_SIZE, and CREATE_BITMAP_AREA_SIZE. Setting these parameters can be difficult because the maximum work area size is ideally selected on the basis of the data input size and the total number of work areas active in the system. These two factors vary a lot from one work area to another and from one time to another. Thus, these parameters are hard to tune under the best circumstances.
Differences in memory allocation between Dedicated Server and Shared Server:
The content of the PGA memory varies, depending whether the instance is running in a Dedicated Server or Shared Server configuration. Generally the PGA memory includes these components:
Process Structure
Oracle takes advantage of various types of processes:
User process: Started at the time a database user requests connection to the Oracle server
Server process: Connects to the Oracle Instance and is started when a user establishes a session
Background processes: Started when an Oracle Instance is started
User Process
A program that requests interaction with the Oracle server
Must first establish a connection
Does not interact directly with the Oracle server
User Process
A database user who needs to request information from the database must first make a connection with the Oracle server. The connection is requested using a database interface tool, such as SQL*Plus, and beginning the user process. The user process does not interact directly with the Oracle server. Rather it generates calls through the user program interface (UPI), which creates a session and starts a server process.
Server Process
A program that directly interacts with the Oracle server
Fulfills calls generated and returns results
Can be Dedicated or Shared Server
Server Process
Once a user has established a connection, a server process is started to handle the user processes requests. A server process can be either a Dedicated Server process or a Shared Server process. In a Dedicated Server environment, the server process handles the request of a single user process. Once a user process disconnects, the server process is terminated. In a Shared Server environment, the server process handles the request of several user processes. The server process communicates with the Oracle server using the Oracle Program Interface (OPI).
Note: Allocation of server process in a dedicated environment versus a shared environment is covered in further detail in the Oracle9i Database Performance Tuning course.
Background Processes
Maintains and enforces relationships between physical and memory structures
Mandatory background processes:
DBWn PMON CKPT
LGWR SMON
Optional background processes:
ARCn LMDn RECO
CJQ0 LMON Snnn
Dnnn Pnnn
LCKn QMNn
Background Processes
The Oracle architecture has five mandatory background processes that are discussed further in this lesson. In addition to the mandatory list, Oracle has many optional background process that are started when their option is being used. These optional processes are not within the scope of this course, with the exception of the background process, ARCn. Following is a list of some optional background processes:
RECO: Recoverer
QMNn: Advanced Queuing
ARCn: Archiver
LCKn: RAC Lock Manager–Instance Locks
LMON: RAC DLM Monitor–Global Locks
LMDn: RAC DLM Monitor–Remote Locks
CJQ0: Coordinator Job Queue background process
Dnnn: Dispatcher
Snnn: Shared Server
Pnnn: Parallel Query Slaves
Database Writer (DBWn)
DBWn writes when:
Checkpoint occurs
Dirty buffers reach threshold
There are no free buffers
Timeout occurs
RAC ping request is made
Tablespace OFFLINE
Tablespace READ ONLY
Table DROP or TRUNCATE
Tablespace BEGIN BACKUP
Database Writer (DBWn)
The server process records changes to undo and data blocks in the Database Buffer Cache. DBWn writes the dirty buffers from the Database Buffer Cache to the datafiles. It ensures that a sufficient number of free buffers (buffers that can be overwritten when server processes need to read in blocks from the datafiles) are available in the Database Buffer Cache. Database performance is improved because server processes make changes only in the Database Buffer Cache.
DBWn defers writing to the datafiles until one of the following events occurs:
Incremental or normal checkpoint
The number of dirty buffers reaches a threshold value
A process scans a specified number of blocks when scanning for free buffers and cannot find any
Timeout occurs
A ping request in Real Application Clusters (RAC) environment
Placing a normal or temporary tablespace offline
Placing a tablespace in read-only mode
Dropping or truncating a table
ALTER TABLESPACE tablespace name BEGIN BACKUP
Log Writer (LGWR)
LGWR writes:
At commit
When one-third full
When there is 1 MB of redo
Every three seconds
Before DBWn writes
Log Writer (LGWR)
LGWR performs sequential writes from the Redo Log Buffer to the redo log file under the following situations:
When a transaction commits
When the Redo Log Buffer is one-third full
When there is more than 1 MB of changes recorded in the Redo Log Buffer
Before DBWn writes modified blocks in the Database Buffer Cache to the datafiles
Every three seconds
Because the redo is needed for recovery, LGWR confirms the commit operation only after the redo is written to disk.
LGWR can also call on DBWn to write to the datafiles.
System Monitor (SMON)
Responsibilities:
Instance recovery
Rolls forward changes in redo logs
Opens database for user access
Rolls back uncommitted transactions
Coalesces free space
Deallocates temporary segments
System Monitor (SMON)
If the Oracle Instance fails, any information in the SGA that has not been written to disk is lost. For example, the failure of the operating system causes an instance failure. After the loss of the instance, the background process SMON automatically performs instance recovery when the database is reopened. Instance recovery consists of the following steps:
1. Rolling forward to recover data that has not been recorded in the datafiles but that has been recorded in the online redo log. This data has not been written to disk because of the loss of the SGA during instance failure. During this process, SMON reads the redo log files and applies the changes recorded in the redo log to the data blocks. Because all committed transactions have been written to the redo logs, this process completely recovers these transactions.
2. Opening the database so that users can log on. Any data that is not locked by unrecovered transactions is immediately available.
3. Rolling back uncommitted transactions. They are rolled back by SMON or by the individual server processes as they access locked data.
SMON also performs some space maintenance functions:
It combines, or coalesces, adjacent areas of free space in the datafiles.
It deallocates temporary segments to return them as free space in datafiles.
Process Monitor (PMON)
Cleans up after
failed processes by:
Rolling back the transaction
Releasing locks
Releasing other resources
Restarting dead dispatchers
.Process Monitor (PMON)
The background process PMON cleans up after failed processes by:
Rolling back the user’s current transaction
Releasing all currently held table or row locks
Freeing other resources currently reserved by the user
Restarts dead dispatchers
Dispatchers are covered in further detail in the Oracle9i Database Administration Fundamentals II course.
Checkpoint (CKPT)
Responsible for:
Signaling DBWn at checkpoints
Updating datafile headers with checkpoint information
Updating control files with checkpoint information
Checkpoint (CKPT)
Every three seconds the CKPT process stores data in the control file to identify that place in the redo log file where recovery is to begin, this being called a checkpoint. The purpose of a checkpoint is to ensure that all of the buffers in the Database Buffer Cache that were modified prior to a point in time have been written to the datafiles. This point in time (called the checkpoint position) is where database recovery is to begin in the event of an instance failure. DBWn will already have written all of the buffers in the Database Buffer Cache that were modified prior to that point in time. Prior to Oracle9i, this was done at the end of the redo log. In the event of a log switch CKPT also writes this checkpoint information to the headers of the datafiles.
Checkpoints are initiated for the following reasons:
To ensure that modified data blocks in memory are written to disk regularly so that data is not lost in case of a system or database failure
To reduce the time required for instance recovery. Only the redo log entries following the last checkpoint need to be processed for recovery to occur
To ensure that all committed data has been written to the datafiles during shutdown
Checkpoint information written by CKPT includes checkpoint position, system change number, location in the redo log to begin recovery, information about logs, and so on.
Note: CKPT does not write data blocks to disk or redo blocks to the online redo logs.
Archiver (ARCn)
Optional background process
Automatically archives online redo logs when ARCHIVELOG mode is set
Preserves the record of all changes made to the database
Archiver (ARCn)
All other background processes are optional, depending on the configuration of the database; however, one of them, ARCn, is crucial to recovering a database after the loss of a disk. As online redo log files get filled, the Oracle server begins writing to the next online redo log file. The process of switching from one redo log to another is called a log switch. The ARCn process initiates backing up, or archiving, of the filled log group at every log switch. It automatically archives the online redo log before the log can be reused, so that all of the changes made to the database are preserved. This enables the DBA to recover the database to the point of failure even if a disk drive is damaged.
Archiving redo log files:
One of the important decisions that a DBA has to make is whether to configure the database to operate in ARCHIVELOG or in NOARCHIVELOG mode.
NOARCHIVELOG mode: In NOARCHIVELOG mode, the online redo log files are overwritten each time a log switch occurs. LGWR does not overwrite a redo log group until the checkpoint for that group is complete. This ensures that committed data can be recovered if there is an instance crash. During the instance crash, only the SGA is lost. There is no loss of disks, only memory. For example, an operating system crash causes an instance crash.
ARCHIVELOG mode: If the database is configured to run in ARCHIVELOG mode, inactive groups of filled online redo log files must be archived before they can be used again. Since changes made to the database are recorded in the online redo log files, the database administrator can use the physical backup of the datafiles and the archived online redo log files to recover the database without losing any committed data because of any single point of failure, including the loss of a disk. Usually, a production database is configured to run in ARCHIVELOG mode.
Archive log modes are covered in further detail in the Oracle9i Database Administration Fundamentals II course.
Logical Structure
Dictates how the physical space of a database is used
Hierarchy consisting of tablespaces, segments, extents, and blocks
Logical Structure
A logical structure hierarchy exists as follows:
An Oracle database contains at least one tablespace.
A tablespace contains one or more segments.
A segment is made up of extents.
An extent is made up of logical blocks.
A block is the smallest unit for read and write operations.
The Oracle database architecture includes logical and physical structures that make up the database.
The physical structure includes the control files, online redo log files, and datafiles that make up the database.
The logical structure includes tablespaces, segments, extents, and data blocks.
The Oracle server enables fine-grained control of disk space use through tablespace and logical storage structures, including segments, extents, and data blocks.
Tablespaces:
The data in an Oracle database are stored in tablespaces.
An Oracle database can be logically grouped into smaller logical areas of space known as tablespaces.
A tablespace can belong to only one database at a time.
Each tablespace consists of one or more operating system files, which are called datafiles.
A tablespace may contains one or more segments.
Tablespaces can be brought online while the database is running.
Except for the SYSTEM tablespace or a tablespace with an active undo segment, tablespaces can be taken offline, leaving the database running.
Tablespaces can be switched between read-write and read-only status.
Datafiles (Not a logical structure):
Each tablespace in an Oracle database consists of one or more files called datafiles. These are physical structures that conform with the operating system on which the Oracle server is running.
A data file can belong to only one tablespace.
An Oracle server creates a data file for a tablespace by allocating the specified amount of disk space plus a small amount of overhead.
The database administrator can change the size of a data file after its creation or can specify that a data file should dynamically grow as objects in the tablespace grow.
Segments:
A segment is the space allocated for a specific logical storage structure within a tablespace.
A tablespace may consist of one or more segments.
A segment cannot span tablespaces; however, a segment can span multiple datafiles that belong to the same tablespace.
Each segment is made up of one or more extents.
Extents:
Space is allocated to a segment by extents.
One or more extents make up a segment.
When a segment is created, it consists of at least one extent.
As the segment grows, extents get added to the segment.
The DBA can manually add extents to a segment.
An extent is a set of contiguous Oracle blocks.
An extent cannot span datafiles, and therefore, it must exist in one datafile.
Data blocks:
The Oracle server manages the storage space in the datafiles in units called Oracle blocks or data blocks.
At the finest level of granularity, the data in an Oracle database is stored in data blocks.
Oracle data blocks are the smallest units of storage that the Oracle server can allocate, read, or write.
One data block corresponds to one or more operating system blocks allocated from an existing data file.
The standard data block size for an Oracle database is specified by the DB_BLOCK_SIZE initialization parameter when the database is created.
The data block size should be a multiple of the operating system block size to avoid unnecessary I/O.
The maximum data block size is dependent on the operating system.
Processing SQL Statements
Connect to an instance using:
User process
Server process
The Oracle server components that are used depend on the type of SQL statement:
Queries return rows
DML statements log changes
Commit ensures transaction recovery
Some Oracle server components do not participate in SQL statement processing
Processing SQL Statements
Processing a query:
Parse:
Search for identical statement
Check syntax, object names, and privileges
Lock objects used during parse
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:
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.
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.
The data blocks record the new values of the data.
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.
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.
Summary
In this lesson, you should have learned how to:
Explain database files: datafiles, control files, online redo logs
Explain SGA memory structures: Database Buffer Cache, Shared Pool, and Redo Log Buffer
Explain primary background processes: DBWn, LGWR, CKPT, PMON, SMON
Explain the use of the background process ARCn
Identify optional and conditional background processes
Explain logical hierarchy
Practice 1 Overview
This practice covers the following topics:
Review of architectural components
Structures involved in connecting a user to an Oracle Instance
Practice 1: Oracle Architectural Components
1 Which one of the following statements is true?
a An Oracle server is a collection of data consisting of three file types.
b A user establishes a connection with the database by starting an Oracle Instance.
c A connection is a communication pathway between the Oracle server and the
Oracle Instance.
d A session starts when a user is validated by the Oracle server.
2 Which one of the following memory areas is not part of the SGA?
a Database Buffer Cache
b PGA
c Redo Log Buffer
d Shared Pool
3 Which two of the following statements are true about the Shared Pool?
a The Shared Pool consists of the Library Cache, Data Dictionary Cache, Shared SQL area, Java Pool, and Large Pool.
b The Shared Pool is used to store the most recently executed SQL statements.
c The Shared Pool is used for an object that can be shared globally.
d The Library Cache consist of the Shared SQL and Shared PL/SQL areas.
4 Which one of the following memory areas is used to cache the data dictionary
information?
a Database Buffer Cache
b PGA
c Redo Log Buffer
d Shared Pool
5 The primary purpose of the Redo Log Buffer is to record all changes to the
database data blocks.
a True
b False
6 The PGA is a memory region that contains data and control information for multiple
server processes or multiple background processes.
a True
b False
7 Which of the following becomes available when an Oracle Instance is started?
a User process
b Server process
c Background processes
8 Identify five mandatory background processes.
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9 Match the process with its task.
a Database Writer E Assists with writing to the data file headers
b Log Writer C Responsible for instance recovery
c System Monitor D Cleans up after failed processes
d Process Monitor B Records database changes for recovery purposes
e Checkpoint A Writes dirty buffers to the datafiles
10 The physical structure of an Oracle database consists of control files, datafiles, and
redo log files.
a True
b False
11 Place the following structures in order of hierarchy beginning with database.
a Tablespaces
b Extent
c Segment
d Database
e Block
12 Identify the components of an Oracle server.
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13 Identify the components of an Oracle Instance.
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14 Identify three file types that make up an Oracle database.
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