The Oracle8 RDBMS for NT is written using Microsoft’s 32-bit API. By using the Microsoft 32-bit API the Oracle8 RDBMS has been tightly integrated with the underlying hardware. Oracle8’s architecture for Microsoft Windows NT has been implemented as a single multithreaded process to conform with the Windows NT memory model.Under the Windows NT operating system a process represents a logical unit of work or job that the operating system is to perform. A thread is one of many subtasks that are required to accomplish the job. The components of a thread include:
A unique identifier called a client ID.
The content of a set of registers that represent the state of the processor.
A stack for when the thread is running in user mode and a stack for when the thread is running in kernel mode.
The thread resides within the process’s virtual address space. When more then one thread exists in the same process, the threads share the address space of all the process’s resources. The NT kernel schedules processes thread(s) for execution. All processes running under Windows NT must have at least one thread before the process can be executed.Unlike Oracle8 for UNIX, Oracle8 for NT uses a single process with multiple threads, thereby sharing memory in a single address space. The database uses the operating system facility for preemptive scheduling and load balancing across multiple CPUs.The Oracle instance on Windows NT consists of a memory segment and a number of background threads. By default, the Oracle8 server and its associated background threads run in the Normal Priority class. In this class, the scheduler can dynamically vary the priority between 1 and 15, but it cannot raise the dynamic priority to the real-time priority class. The real-time priority class ranges from 16 to 31 and cannot vary in priority based on behavior.The Windows NT GUI and its associated utilities can be used to observe various portions of the Oracle8 RDBMS. The Windows menu is used to access the Windows NT control panel.By accessing the Windows NT CONTROL PANEL the administrator can perform various tasks One such task may be to observe, start, or stop any of the various services that are running on the machine. The various services associated with the Oracle RDBMS. The remainder of this chapter is used to investigate the various components of the Oracle8 RDBMS architecture. Where possible the GUI utilities provided by the Windows NT operating system are used to observe the various components of the RDBMS. The same utilities will also help develop our understanding of how the Oracle8 RDBMS is integrated with the Windows NT operating system.ORACLE8 RDBMS ARCHITECTUREThe architecture of the Oracle RDBMS is divided into two distinct parts. One part is called the Oracle database the other part is called the Oracle The Oracle database is defined as:
A logical collection of data to be treated as a unit (tables).
Operating system files called data files, redo log files, initialization files and control files.
The Oracle instance is defined as:
The software mechanism used for accessing and controlling the database.
Having at least four background threads called PMON, SMON, DBWR and LGWR.
Including memory structures called the SGA and the PGA.
Each Oracle instance is identified by a System Identifier (SID).
Instances and databases are independent of each other, but neither is of any use without the other. For the end user to access the database the Oracle instance must be started (the four background threads must be running) and the database must be mounted (by the instance) and opened. In the simple model a database can be mounted by only one instance. The exception to this is the Oracle parallel server, where a database can be mounted by more then one Oracle instance.ORACLE DATABASE STRUCTUREOur discussion of the Oracle RDBMS architecture will first focus on that part that makes up the Oracle database. The Oracle database has both a physical and a logical structure. The physical structure consists of the operating system files that make up the database. The logical structure is determined by the number of tablespaces and the database’s schema objects.TablespacesAll Oracle databases must consist of at least one logical entity called a tablespace. The characteristics of a tablespace are:
One or more per database. The database must have at least one tablespace called “SYSTEM.” The SYSTEM tablespace holds the Oracle Data Dictionary. The Data Dictionary holds the various system tables and views such as the Oracle performance tables, information about the users of the database, and how much space is left in the various tablespaces that make up the database. There are usually more tablespaces other than the SYSTEM tablespace. Most Oracle databases also include additional tablespaces. These tablespaces are used to hold user data for sorting, and indexes that are used to speed up data access. Additional tablespaces should be created to hold data that is being sorted and another tablespace to hold data that is required for read consistency.
The physical representation of the tablespace is called a data file (a tablespace may consist of more then one data file).
Can be taken off line (due to media failure or maintenance purposes) leaving the database running. The exception to this rule is that the SYSTEM tablespace cannot be taken off line if the database is to remain running.
Unit of space for object storage. Objects are tables, indexes synonyms, and clusters.
Contains default storage parameters for database objects.
When an end-user’s Oracle user ID is created the user is given access to a default tablespace and a temporary tablespace (where the sorting of data is performed).
Can be dropped (removed from the database).
As stated previously tablespaces are logical entities. Tablespaces are physically represented by files that are called data files. Data files have the following attributes:
Are operating system files.
There is one or more per tablespace.
The finest granularity of the data file is called the data block.
A collection of data blocks is called an extent.
A segment (by definition) consists of one or more extents (therefore to make a segment larger, extents are added to the segment).
A data file consists of segments.
Contain transaction System Change Numbers (SCNs).
Data File Contents and Types of SegmentsA data file can consist of several types of segments and a segment can consist of one of more extents. The four different types of segments are rollback segments, temporary segments, index segments and data segments.Rollback segments have the following attributes:
Records old data.
Provides for rollback of uncommitted transactions.
Provides information for read consistency.
Used during database recovery from media or processor failure.
Wrap-around/reuseable.
Can be dynamically created or dropped.
Rollback segments contain the following information:
Transaction ID.
File ID.
Block number
Row number
Column number
Row/column data.
Temporary segments have the following attributes:
Used by the Oracle RDBMS as a work area for sorting data.
The DBA defines which tablespace will contain temporary segments and therefore the tablespace where sorting will occur.
Index segments have the following attributes:
Allows for faster data retrieval by providing an index for the data in a table, thus eliminating a full table scan during the execution of a query (similar to how a reader would use the index in a book rather then scanning through the entire book to find a particular topic).
Data segments have the following attributes:
One per table/snapshot.
Contains all table data.
Data segments contain the following information:
Transaction ID.
File ID.
Block number
Row number
Column number
Row/column data.
Besides data files there are also files called redo log files. Redo log files record changes made to the database by various transactions. All changes made to the database will first be written to the redo log file. These files can also be written to an off-line log file (archived). Redo logs are used during database recovery to recover the database to the last physical backup or to the point in time of failure (for this type of recovery the database must be running in ARCHIVELOG mode). Redo log files have the following attributes:
Records new data.
Ensures permanence of data transactions.
Provides for roll forward recovery during database startup and after a media failure.
Redo log files contain:
Transaction IDs
Contents of redo log buffers.
Transaction SCN.
The Control FileEach database has one or more control files. The control file is used to store information about the database. The information in the control file includes:
Transaction System Change Number (SCN)
Location of all datafiles.
Names and locations of the redo log files.
Time stamp when database was created.
Database name.
Database size.
For database recovery purposes it is best to have multiple copies of the control file. Without the control file the Oracle RDBMS cannot find the pointers to the rest of the files that make up the database (data files and redo log files).The INIT
The maximum number of processes that the Oracle instance will use (PROCESSES=).
The name of the database (DB_NAME=).
Various parameters for tuning memory management (DB_BLOCK_BUFFERS, SORT_AREA_SIZE...)
The location of the control file(s).
How these parameters affect the starting and running of the database are covered in the chapters on Oracle RDBMS installation and performance analysis and tuning.
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