Free Oracle Essentials Oracle Database 11g by Rick Greenwald Page B

island exists but no traffic flow is possible. In Oracle terms, if the instance is up, data can flow in and out of the database. The physical state of the database is changing. If the instance is down, users cannot access the database even though it still exists physically. The database is static: no changes can occur to it. When the instance comes back into service, the data will be there waiting for it.
    Oracle Database Structure
    Oracle’s database structures include tablespaces, control files, redo log files, archived logs, block change tracking files, Flashback logs, and recovery backup (RMAN) files.
    This section introduces many of the structures and other components that make up a complete database.
    Tablespaces
    All of the data stored within an Oracle database must reside in a tablespace. A tablespace is a logical structure; you can’t look at the operating system and see a tablespace. Each tablespace is composed of physical structures called datafiles ; each tablespace must consist of one or more datafiles, and each datafile can belong to only one tablespace. When creating a table, youcan specify the tablespace in which to create it. Oracle will then find space for it in one of the datafiles that make up the tablespace.
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    Chapter 2: Oracle Architecture

    Figure 2-2 shows the relationship of tablespaces to datafiles for a database.
    DATA1
    DATA2
    data1_01.dbf
    data2_01.dbf
    data2_02.dbf
    The DATA1
    The DATA2
    tablespace consists
    tablespace consists
    of one datafile
    of two datafiles
    Figure 2-2. Tablespaces and datafiles
    This figure shows two tablespaces within an Oracle database. When you create a new table in this Oracle database, youmay place it in the DATA1 tablespace or the DATA2 tablespace. It will physically reside in one of the datafiles that make up the specified tablespace.
    Oracle’s default tablespaces for all types of tables are locally managed tablespaces as of Oracle Database 10 g Release 2. Locally managed tablespaces enable creation of bigfile tablespaces that can leverage 64-bit systems and their ability to manage ultra-large files.
    Oracle9 i introduced the concept of Oracle Managed Files (OMFs), which enable your database to automatically create, name, and delete, where appropriate, all the files that make up your database. OMFs reduce the maintenance overhead of naming and tracking the filenames for your database, as well as avoiding the problems that can result from human errors in performing these tasks. Since Oracle Database 10 g , OMFs and bigfile tablespaces combine to make datafiles appear completely transparent.
    Oracle databases can be deployed on up to 64,000 datafiles. Because a bigfile tablespace can contain a file that is 1,024 times larger than a smallfile tablespace, and bigfile tablespaces have 32 KB block sizes on 64-bit operating systems, the Oracle database can grow to up to 8 exabytes in size (an exabyte is equivalent to a million terabytes).* The bigfile tablespace is designed for use with Oracle’s Automatic Storage Management (ASM), other logical volume managers that support striping, and RAID.†
    * The ultimate size of a bigfile depends on the limitations of the underlying operating system.
    † RAID stands for “redundant array of inexpensive disks” and is described in Chapter 7.
    Databases and Instances
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    Files of a database
    There are actually three fundamental types of physical files that make up an Oracle database:
    • Control files
    • Datafiles
    • Redo log files
    These three fundamental types represent the physical database itself. Figure 2-3 illustrates the three types of files and their interrelationships.
    Control
    Files
    Identify
    Identify
    Record changes to
    Datafiles
    Redo Log Files
    Figure 2-3. The files that make up a database
    The control file contains locations for other physical files that form the database: the datafiles and redo log files. It also contains key information about the contents and state of the database,