The Disk Component

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The Disk Component

The database server uses the physical units of storage to allocate disk space. You define the logical units that the database server uses to store data. All the databases and all the system information that is necessary to maintain the database server reside within the disk component.

On UNIX, the database server stores data in two types of disk space: raw and cooked. The database server allows you to use either type of disk space or a combination of both types.

Raw disk space (also called unbuffered disk space) is unformatted space where the database server manages the physical organization of the data.
Cooked disk space (also called buffered disk space) refers to regular operating-system files.

On Windows, the database server stores data in two types of disk space:

New Technology File System (NTFS)
Logical partition or physical drive

The database server uses the following physical units to manage disk space.

Physical Unit	Description
Chunk	The largest unit of database server data storage
Page	The physical unit of disk storage to read from and write to databases
Blobpage	The physical unit of disk storage to store simple large objects in a blobspace
Sbpage	The physical unit of disk storage to store smart large objects in an sbspace
Extent	A fixed amount of space to contain the data stored in a table

The database server uses the following logical units to manage disk space. Dbspaces, blobspaces, and sbspaces are composed of one or more chunks.

Logical Storage Unit	Description
Dbspace	Stores databases, tables, logical-log files, the physical log, and internal data
Blobspace	Stores simple large objects (TEXT and BYTE data)
Sbspace	Stores smart large objects (CLOB and BLOB data)
Extspace	References the location of external data
Database	Contains tables and indexes
Table	Consists of a row of column headings with zero or more rows of data values
Tblspace	Contains the disk space allocated to a given table or fragment

The database server maintains the following storage structures to ensure physical and logical data consistency.

Data Consistency	Description
Logical log	A circular file that stores log records of transactions and database server changes
Physical log	A set of disk pages where the database server stores an unmodified copy of the page (called a before image)

For information about storage spaces and logical and physical logs, see the IBM Informix Administrator's Guide. Logical-log record formats are discussed in the IBM Informix Administrator's Reference.