Exploring the Database Architecture
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all the redo generated since the backup was made The necessary redo is extracted from the change vectors in the online and archive redo log files The routines for datafile backup, restore, and recovery are described in s 16 and 17
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Other Database Files
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These files exist externally to the database They are, for practical purposes, necessary but they are not strictly speaking part of the database
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The Instance Parameter File
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When an Oracle instance is started, the SGA structures build in memory and the background processes start according 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 All others have defaults So the parameter file can be quite small, but it must exist Creating the parameter file is described in 4 Users establish sessions by presenting a username and a password The Oracle server authenticates these against user definitions stored in the data dictionary The data dictionary is a set of tables in the database; it is therefore inaccessible if the database is not open There are occasions when a user needs to be authenticated before the data dictionary is available: when he needs to start the database, or indeed to create it An external password file is one means of doing this It contains a small number (typically less than half a dozen) of user names and passwords that exist outside the data dictionary, and which can therefore be used to connect to an instance before the data dictionary is available Creating the password file is described in 4 When an online redo log file fills, the ARCn process copies it out of the database to an archive redo log file Once this is done, the archive log is no longer part of the database It is, however, essential if it is ever necessary to restore a datafile backup, and Oracle does provide facilities for managing the archive redo log files The alert log is a continuous stream of messages regarding certain critical operations affecting the instance and the database Not everything is logged: only events that are considered to be really important, such as startup and shutdown; changes to the physical structures of the database; changes to the parameters that control the instance Trace files are generated by background processes when they detect error conditions, and sometimes to report certain actions
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The Password File
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Archive Redo Log Files
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Alert Log and Trace Files
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Summarize the Storage Structures
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SCENARIO & SOLUTION
You are taking over management of a database, and there is no documentation of the physical or logical storage, or the memory usage How can you work out how it is configured You must run queries against numerous views that describe the database and its contents There are graphical tools that will help many are included with Oracle Enterprise Manager but all these are doing is running queries against views, and formatting the results in a pretty fashion No, it doesn t but you may need some help In principle, the Oracle architecture is identical on all platforms, and when you are working from within an Oracle tool such as SQL*Plus or SQL Developer, it really doesn t matter what the platform is That having been said, unless you are reasonably competent at working from the operating system prompt as well, you will need a lot of help from the system administrators Teamwork! A vital skill in IT support
Worse still, the database is on an operating system and hardware with which you are not familiar Does that mean you can t manage it
The Logical Database Structures
The physical structures that make up a database are visible as operating system files to your system administrators Your users see logical structures such as tables Oracle uses the term segment to describe any structure that contains data A typical segment is a table, containing rows of data, but there are more than a dozen possible segment types in an Oracle database Of particular interest (for examination purposes) are table segments, index segments, and undo segments, all of which are investigated in detail later on For now, you don t need to know more than that tables contain rows of information; that indexes are a mechanism for giving fast access to any particular row; and that undo segments are data structures used for storing the information that might be needed to reverse, or roll back, any transactions that you do not wish to make permanent So system administrators see physical datafiles; programmers see logical segments Oracle abstracts the logical storage from the physical storage by means of the tablespace A tablespace is logically a collection of one or more segments, and physically a collection of one or more datafiles Put in terms of relational analysis,