tablespaces and datafiles in database administration

Database Administrator
Lecture 10

Managing Tablespaces and Data
files

Objectives
 After completing this lesson, you should be able to do
the following:
 Describe the logical structure of the database
 Create tablespaces
 Change the size of tablespaces
 Allocate space for temporary segments
 Change the status of tablespaces
 Change the storage settings of tablespaces
 Implement Oracle Managed Files

Overview
 A small database might need only the SYSTEM tablespace
 Oracle recommends that you create additional tablespaces
to store user data, user indexes, undo segments, and
temporary segments separate from data dictionary
 The DBA can create new tablespaces, resize data files, add
data files to tablespaces
 Set and alter default segment storage settings for segments
created in a tablespace, make a tablespace read-only or
read-write, make a tablespace temporary or permanent, and
drop tablespaces

Database Architecture
 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 data files 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 data files
 A tablespace may consist of zero or more segments
 Tablespaces can be brought online while the database is running
 Tablespaces can be switched between read-write and read-only
stat

Data Files
 Each tablespace in an Oracle database consists of one or
more files called data files. 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. For example, all of the
storage allocated to a table is a segment.
 A tablespace may consist of one or more segments.
 A segment cannot span tablespaces; however, a segment
can span multiple data files 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 a data file but must exist in one data file

Data Blocks
 The Oracle server manages the storage space in the data files in
units called Oracle blocks or data blocks.
 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

SYSTEM and Non-SYSTEM
Tablespaces
 SYSTEM tablespace:
 Created with the database
 Required in all databases
 Contains the data dictionary, including stored program units
 Contains the SYSTEM undo segment
 Should not contain user data, although it is allowed
 Non-SYSTEM tablespaces:
 Enable more flexibility in database administration
 Separate undo, temporary, application data, and application index
segments
 Separate data by backup requirements
 Separate dynamic and static data
 Control the amount of space allocated to user’s objects

Creating Tablespaces
 CREATE TABLESPACE tablespace
[DATAFILE clause]
[MINIMUM EXTENT integer[K|M]]
[BLOCKSIZE integer [K]]
[LOGGING|NOLOGGING]
[DEFAULT storage_clause ]
[ONLINE|OFFLINE]
[PERMANENT|TEMPORARY]

 where:
 tablespace is the name of the tablespace to be created
 DATAFILE specifies the data file or data files that make up the
tablespace
 MINIMUM EXTENT ensures that every used extent size in the
tablespace is a multiple of the integer. Use Kor M to specify this
size in kilobytes or megabytes.
 LOGGING specifies that, by default, all tables, indexes, and
partitions within the tablespace have all changes written to redo.
LOGGING is the default

 NOLOGGING specifies that, by default, all tables, indexes, and
partitions within the tablespace do not have all changes written to
redo. NOLOGGING affects only some DML and DDL commands, for
example, direct loads.
 DEFAULT specifies the default storage parameters for all objects
created in the tablespace creation
 OFFLINE makes the tablespace unavailable immediately after creation
 PERMANENT specifies that the tablespace can be used to hold
permanent objects
 TEMPORARY specifies that the tablespace be used only to hold
temporary objects; for example, segments used by implicit sorts
caused by an ORDER BY clause extent_management_clause
specifies how the extents of the tablespace are managed. This clause
is discussed in a subsequent section of this lesson

 CREATE TABLESPACE userdata
DATAFILE '/u01/oradata/userdata01.dbf' SIZE 100M
AUTOEXTEND ON NEXT 5M MAXSIZE 200M;
 where:
 filename is the name of a data file in the tablespace
 SIZE specifies the size of the file. Use K or M to specify the size
in kilobytes or megabytes.
 REUSE allows the Oracle server to reuse an existing file
 autoextend_clause enables or disables the automatic extension
of the data file.

Space Management in
Tablespaces
 Locally managed tablespaces:
 Free extents recorded in bitmap
 Each bit corresponds to a block or group of blocks
 Bit value indicates free or used
 Dictionary-managed tablespaces:
 Default method
 Free extents recorded in data dictionary tables

Advantages of Locally Managed
Tablespaces
 Local management avoids recursive space management operations, which
can occur in dictionary-managed tablespaces if consuming or releasing
space in an extent results in another operation that consumes or releases
space in a undo segment or data dictionary table.
 Because locally managed tablespaces do not record free space in data
dictionary tables, it reduces contention on these tables.
 Local management of extents automatically tracks adjacent free space,
eliminating the need to coalesce free extents.
 The sizes of extents that are managed locally can be determined
automatically by the system. Alternatively, all extents can have the same
size in a locally managed tablespace.
 Changes to the extent bitmaps do not generate undo information because
they do not update tables in the data dictionary

Locally Managed Tablespaces
 CREATE TABLESPACE userdata
DATAFILE '/u01/oradata/userdata01.dbf' SIZE 500M
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 256K;
 The LOCAL option of the EXTENT MANAGEMENT clause
specifies that a tablespace is to be locally managed.
 By default a tablespace is locally managed
 extent_management_clause :==
[ EXTENT MANAGEMENT
[ DICTIONARY | LOCAL
[ AUTOALLOCATE | UNIFORM [SIZE integer[K|M]] ] ] ]

Dictionary Managed Tablespaces
 Extents are managed in the data dictionary
 Each segment stored in the tablespace can have a
different storage clause
 Merging required

tablespaces and datafiles in database administration

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