IBM 000-8697 User Manual

IBM Informix OnLine Database Server Administrator’s Guide

This same unlikely scenario could occur during the rollback of a long trans-
action even after the

LTXEHWM

 is reached. (Refer to

 for more

details about the long transaction exclusive high-water mark.) After the

LTXEHWM

 is reached, and after all processes have exited critical sections,

only the database server process that is performing the rollback has access to
the physical and logical logs. However, if many processes were in critical
sections, and if the space remaining in the physical log were very small at the
time the

LTXEHWM

was reached, it is conceivable that the writes performed

as user processes completed their processing could fill the physical log
during the rollback.

The function of the logical log is to store a record of changes to OnLine data
since the last OnLine archive. OnLine manages the logical log as three or
more separate allocations of disk space, each of which is referred to as a
logical log file. Each logical log file is associated with a unique identification
number.

Refer to

 for a listing of logical log administration topics.

The logical log records can be applied to the OnLine system to recover all
transactions that occurred since the most-recent point of known physical
consistency. The point of known consistency in an OnLine database server
system is called a checkpoint. The logical log records are used in the second
phase of fast recovery when OnLine returns the entire system to a state of
logical consistency up to the point of the most-recent logical log record.

For further information about the role of the logical log in fast recovery, refer
to

. For further information about checkpoints, refer to

.

For further information about how to display the logical log records, refer to

Backup tapes of the logical log files can be combined with the most-recent
OnLine archives to re-create the OnLine system up to the point of the most-
recent logical log record.