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2-86 IBM Informix OnLine Database Server Administrator’s Guide
OnLine Disk Space Terms and Definitions
Extents for a single table can be located within different chunks of the same
dbspace. However, extents must be located wholly in one chunk or another;
extents cannot span chunk boundaries. All data within an extent pertains to
a single tblspace.
The initial extent of a table and all subsequent “next” extents may differ in
size. The size of the table extents are specified as part of the SQL statement
CREATE TABLE.
Physical Log
The physical log is a unit of physically contiguous disk pages that contain
“before-images” of pages that have been modified during processing. When
the physical log “before-images” are combined with the most-recent records
stored in the logical logs, OnLine can return all data to physical and logical
consistency, up to the point of the most-recently completed transaction. This
is the concept of fast recovery. Refer to page 4-39 for more information about
fast recovery.
Logical Log
The logical log disk space is composed of three or more allocations of physi-
cally contiguous disk pages. Each allocation of space is called a logical log file.
The logical log contains a record of logical operations performed during
OnLine processing. If a database is created with transactions, all transaction
information is stored in the logical log files. Refer to page 3-13 for more infor-
mation about the administrative aspects of the logical log. Refer to page 4-18
for more information about the role of the logical log in logging operations.
(Refer to page 2-153 for more information about the structure and contents of
the logical log files.)
When the logical log record of operations is combined with the archive tapes
of OnLine data, the OnLine data can be restored up to the point of the most-
recently stored logical log record. This is the concept of a data restore. (Refer
to page 4-45 for more information about the data restore procedure.)