The Oracle database has a logical layer and a physical layer. The physical layer consists of the files that reside on the disk; the components of the logical layer map the data to these physical components.
The Physical Layer
The physical layer of the database consists of three types of files:
One or more datafiles--Datafiles store the information contained in the database. You can have as few as one datafile or as many as hundreds of datafiles. The information for a single table can span many datafiles or many tables can share a set of datafiles. Spreading tablespaces over many datafiles can have a significant positive effect on performance. The number of datafiles that can be configured is limited by the Oracle parameter MAXDATAFILES.
Two or more redo log files--Redo log files hold information used for recovery in the event of a system failure. Redo log files, known as the redo log, store a log of all changes made to the database. This information is used in the event of a system failure to reapply changes that have been made and committed but that might not have been made to the datafiles. The redo log files must perform well and be protected against hardware failures (through software or hardware fault tolerance). If redo log information is lost, you cannot recover the system.
One or more control files--Control files contain information used to start an instance, such as the location of datafiles and redo log files; Oracle needs this information to start the database instance. Control files must be protected. Oracle provides a mechanism for storing multiple copies of control files.
The Logical Layer
The logical layer of the database consists of the following elements:
One or more tablespaces.
The database schema, which consists of items such as tables, clusters, indexes, views, stored procedures, database triggers, sequences, and so on.
Tablespaces and Datafiles
New Term: The database is divided into one or more logical pieces known as tablespaces. A tablespace is used to logically group data together.
Table--A table, which consists of a tablename and rows and columns of data, is the basic logical storage unit in the Oracle database. Columns are defined by name and data type. A table is stored within a tablespace; often, many tables share a tablespace.
Cluster--A cluster is a set of tables physically stored together as one table that shares a common column. If data in two or more tables is frequently retrieved together based on data in the common column, using a clustered table can be quite efficient. Tables can be accessed separately even though they are part of a clustered table. Because of the structure of the cluster, related data requires much less I/O overhead if accessed simultaneously.
Index--An index is a structure created to help retrieve data more quickly and efficiently (just as the index in this book allows you to find a particular section more quickly). An index is declared on a column or set of columns. Access to the table based on the value of the indexed column(s) (as in a WHERE clause) will use the index to locate the table data.
View--A view is a window into one or more tables. A view does not store any data; it presents table data. A view can be queried, updated, and deleted as a table without restriction. Views are typically used to simplify the user's perception of data access by providing limited information from one table, or a set of information from several tables transparently. Views can also be used to prevent some data from being accessed by the user or to create a join from multiple tables.
Stored procedure--A stored procedure is a predefined SQL query that is stored in the data dictionary. Stored procedures are designed to allow more efficient queries. Using stored procedures, you can reduce the amount of information that must be passed to the RDBMS and thus reduce network traffic and improve performance.
Database trigger--A database trigger is a procedure that is run automatically when an event occurs. This procedure, which is defined by the administrator or developer, triggers, or is run whenever this event occurs. This procedure could be an insert, a deletion, or even a selection of data from a table.
Sequence--The Oracle sequence generator is used to automatically generate a unique sequence of numbers in cache. By using the sequence generator you can avoid the steps necessary to create this sequence on your own such as locking the record that has the last value of the sequence, generating a new value, and then unlocking the record
Segments, Extents, and Data Blocks
Within Oracle, the space used to store data is controlled by the use of logical structures. These structures consist of the following:
Data blocks--A block is the smallest unit of storage in an Oracle database. The database block contains header information concerning the block itself as well as the data.
Extents--Extents consist of data blocks.
Segments--A segment is a set of extents used to store a particular type of data
Segments
An Oracle database can use four types of segments:
Data segment--Stores user data within the database.
Index segment--Stores indexes.
Rollback segment--Stores rollback information used when data must be rolled back.
Temporary segment--Created when a SQL statement needs a temporary work area; these segments are destroyed when the SQL statement is finished. These segments are used during various database operations, such as sorts.
Extents
Extents are the building blocks of segments; in turn, they consist of data blocks. An extent is used to minimize the amount of wasted (empty) storage. As more and more data is entered into tablespaces in your database, the extents used to store that data can grow or shrink as necessary. In this manner, many tablespaces can share the same storage space without preallocating the divisions between those tablespaces.
At tablespace-creation time, you can specify the minimum number of extents to allocate as well as the number of extents to add at a time when that allocation has been used. This arrangement gives you efficient control over the space used in your database.
Data Blocks
Data blocks are the smallest pieces of an Oracle database; they are physically stored on disk. Although the data block in most systems is 2KB (2,048 bytes), you can change this size for efficiency depending on your application or operating system.
The Oracle Instance
The Oracle instance consists of the Oracle processes and shared memory necessary to access information in the database. The instance is made up of the user processes, the Oracle background processes, and the shared memory used by these processes (see Figure Below).
The Oracle Memory Structure
New Term: Oracle uses shared memory for several purposes, including caching of data and indexes as well as storing shared program code. This shared memory is broken into various pieces, or memory structures. The basic memory structures associated with Oracle are the System Global Area (SGA) and the Program Global Area (PGA).
The System Global Area (SGA)
The SGA is a shared memory region that Oracle uses to store data and control information for one Oracle instance. The SGA is allocated when the Oracle instance starts and deallocated when the Oracle instance shuts down. Each Oracle instance that starts has its own SGA. The information in the SGA consists of the following elements, each of which has a fixed size and is created at instance startup:
The database buffer cache--This stores the most recently used data blocks. These blocks can contain modified data that has not yet been written to disk (sometimes known as dirty blocks), blocks that have not been modified, or blocks that have been written to disk since modification (sometimes known as clean blocks). Because the buffer cache keeps blocks based on a most recently used algorithm, the most active buffers stay in memory to reduce I/O and improve performance.
The redo log buffer--This stores redo entries, or a log of changes made to the database. The redo log buffers are written to the redo log as quickly and efficiently as possible. Remember that the redo log is used for instance recovery in the event of a system failure.
The shared pool--This is the area of the SGA that stores shared memory structures such as shared SQL areas in the library cache and internal information in the data dictionary. The shared pool is important because an insufficient amount of memory allocated to the shared pool can cause performance degradation. The shared pool consists of the library cache and the data-dictionary cache.
The Library Cache
The library cache is used to store shared SQL. Here the parse tree and the execution plan for every unique SQL statement are cached. If multiple applications issue the same SQL statement, the shared SQL area can be accessed by each to reduce the amount of memory needed and to reduce the processing time used for parsing and execution planning.
The Data-Dictionary Cache
The data dictionary contains a set of tables and views that Oracle uses as a reference to the database. Oracle stores information here about the logical and physical structure of the database. The data dictionary contains information such as the following:
User information, such as user privileges
Integrity constraints defined for tables in the database
Names and data types of all columns in database tables
Information on space allocated and used for schema objects
The data dictionary is frequently accessed by Oracle for the parsing of SQL statements. This access is essential to the operation of Oracle; performance bottlenecks in the data dictionary affect all Oracle users. Because of this, you should make sure that the data-dictionary cache is large enough to cache this data. If you do not have enough memory for the data-dictionary cache, you see a severe performance degredation. If you ensure that you have allocated sufficient memory to the shared pool where the data-dictionary cache resides, you should see no performance problems.
The Program Global Area (PGA)
The PGA is a memory area that contains data and control information for the Oracle server processes. The size and content of the PGA depends on the Oracle server options you have installed. This area consists of the following components:
Stack space--This is the memory that holds the session's variables, arrays, and so on.
Session information--If you are not running the multithreaded server, the session information is stored in the PGA. If you are running the multithreaded server, the session information is stored in the SGA.
Private SQL area--This is an area in the PGA where information such as binding variables and runtime buffers is kept.
Processes
New Term: In many operating systems, traditional processes have been replaced by threads or lightweight processes. The term process is used in this book to describe a thread of execution, or a mechanism that can execute a set of code; process refers to the mechanism of execution and can refer to a traditional process or a thread.
The Oracle RDBMS uses two types of processes: user processes and Oracle processes (also known as background processes). In some operating systems (such as Windows NT), these processes are actually threads; for the sake of consistency, I will refer to them as processes.
User Processes
User, or client, processes are the user's connections to the RDBMS system. The user process manipulates the user's input and communicates with the Oracle server process through the Oracle program interface. The user process is also used to display the information requested by the user and, if necessary, can process this information into a more useful form.
Oracle Processes
Oracle processes perform functions for users. Oracle processes can be split into two groups: server processes (which perform functions for the invoking process) and background processes (which perform functions on behalf of the entire RDBMS).
Server Processes (Shadow Processes)
Server processes, also known as shadow processes, communicate with the user and interact with Oracle to carry out the user's requests. For example, if the user process requests a piece of data not already in the SGA, the shadow process is responsible for reading the data blocks from the datafiles into the SGA. There can be a one-to-one correlation between user processes and shadow processes (as in a dedicated server configuration); although one shadow process can connect to multiple user processes (as in a multithreaded server configuration), doing so reduces the utilization of system resources.
Background Processes
Background processes are used to perform various tasks within the RDBMS system. These tasks vary from communicating with other Oracle instances and performing system maintenance and cleanup to writing dirty blocks to disk. Following are brief descriptions of the nine Oracle background processes:
DBWR (Database Writer)--DBWR is responsible for writing dirty data blocks from the database block buffers to disk. When a transaction changes data in a data block, that data block need not be immediately written to disk. Therefore, the DBWR can write this data to disk in a manner that is more efficient than writing when each transaction completes. The DBWR usually writes only when the database block buffers are needed for data to be read. Data is written in a least recently used fashion. For systems in which asynchronous I/O (AIO) is available, there should be only one DBWR process. For systems in which AIO is not available, performance can be greatly enhanced by adding more DBWR processes.
LGWR (Log Writer)--The LGWR process is responsible for writing data from the log buffer to the redo log.
CKPT (Checkpoint)--The CKPT process is responsible for signaling the DBWR process to perform a checkpoint and to update all the datafiles and control files for the database to indicate the most recent checkpoint. A checkpoint is an event in which all modified database buffers are written to the datafiles by the DBWR. The CKPT process is optional. If the CKPT process is not present, the LGWR assumes these responsibilities.
PMON (Process Monitor)--PMON is responsible for keeping track of database processes and cleaning up if a process prematurely dies (PMON cleans up the cache and frees resources that might still be allocated). PMON is also responsible for restarting any dispatcher processes that might have failed.
SMON (System Monitor)--SMON performs instance recovery at instance startup. This includes cleaning temporary segments and recovering transactions that have died because of a system crash. The SMON also defragments the database by coalescing free extents within the database.
RECO (Recovery)--RECO is used to clean transactions that were pending in a distributed database. RECO is responsible for committing or rolling back the local portion of the disputed transactions.
ARCH (Archiver)--ARCH is responsible for copying the online redo log files to archival storage when they become full. ARCH is active only when the RDBMS is operated in ARCHIVELOG mode. When a system is not operated in ARCHIVELOG mode, it might not be possible to recover after a system failure. It is possible to run in NOARCHIVELOG mode under certain circumstances, but typically should operate in ARCHIVELOG mode.
LCKn (Parallel Server Lock)--Up to 10 LCK processes are used for interinstance locking when the Oracle Parallel Server option is used.
Dnnn (Dispatcher)--When the Multithreaded Server option is used, at least one Dispatcher process is used for every communications protocol in use. The Dispatcher process is responsible for routing requests from the user processes to available shared server processes and back.
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