IBM 000-8697 User Manual

IBM Informix OnLine Database Server Administrator’s Guide

Logical log records are written continuously during OnLine operation. Even
if a database is not created with transaction logging, administrative changes
(such as adding a dbspace or a chunk) and data definition statements, such
as

CREATE TABLE

or

DROP TABLE

, are logged. (

SELECT

statements are never

logged.) The logical log files contain five types of records:

■

SQL

 data definition statements for all databases

■

SQL

 data manipulation statements for databases that were created

with logging

■

Record of a change to the logging status of a database

■

Record of a checkpoint

■

Record of a change to the configuration

(Refer to

the number and size of logical log records that are written to the logical log
files.)

Three events cause the logical log buffer to flush:

■

One of the logical log buffers becomes full.

■

A transaction is committed within a database that uses unbuffered
logging.

■

A checkpoint occurs.

Refer to

 for a definition of a checkpoint. Refer to

 for a

description of the checkpoint procedure.

If a transaction is committed in a database with unbuffered logging, the
logical log buffer is immediately flushed. This might appear to be a source of
some disk space waste. Typically, many logical log records are stored on a
single page. But because the logical log buffer is flushed in whole pages, even
if only one transaction record is stored on the page, the whole page is flushed.
In the worst case, a single

COMMIT

logical log record (“commit work”) could

occupy a page on disk, and all remaining space on the page would be unused.