
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 physicallog “before-images” are combined with themost-recent records
stored in the logical logs, OnLine can return all data to physical and logical
consistency, up to thepoint of the most-recentlycompleted 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 diskpages.Each allocationofspace is calleda logical logfile.
The logical log contains a record of logical operations performed during
OnLine processing. If a database is created with transactions, all transaction
informationis storedinthe logical logfiles.Refer to page 3-13formoreinfor-
mation about theadministrative aspectsof the logicallog. Referto page 4-18
for more information about the role of the logical log in logging operations.
(Refertopage 2-153formoreinformationaboutthestructureandcontentsof
the logical log files.)
When the logicallogrecord of operationsiscombined with thearchivetapes
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.)
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