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2-82 IBM Informix OnLine Database Server Administrator’s Guide
OnLine Disk Space Terms and Definitions
OnLine maintains three additional disk space structures to ensure physical
and logical consistency of data:
■ A logical log
■ A physical log
■ Reserved pages
Figure 2-11 on page 2-83 illustrates the relationships among these physical
and logical units of disk space. A basic definition of each unit is provided in
the paragraphs that follow.
Chunk
The chunk is the largest unit of physical disk that is dedicated to OnLine data
storage.The chunk can represent an allocation of cooked disk space or raw
disk space.
If a chunk is an allocation of raw disk space, the name of the chunk is the
name of the character-special file in the /dev directory. In many operating
systems, the character-special file can be distinguished from the block-special
file by the letter r, which appears as the first letter in the filename (for
example, /dev/rdsk0a). Space in a chunk of raw disk space is physically
contiguous.
If a chunk is an allocation of cooked disk space, the name of the chunk is the
complete pathname of the cooked file. Since the chunk of cooked disk space
is reserved as an operating-system file, the space in the chunk might or might
not be contiguous.
Page
All space within a chunk is divided into pages. All I/O is done in units of
whole pages. The size of a page is specified as BUFFSIZE in the configuration
file and cannot be changed.