System Architecture 2-155

File Rotation

Forfurther information about what happens during a logical log backup that
makes this possible, refer topage 4-26. For further information about what
happensduring an OnLine restore with archive and logical log backup tapes,
refer topage 4-45.
File Rotation
OnLinesafeguards the logical log records by requiring that a full logical log
file is marked with a status ofused until it is backed up to tape and it is no
longer needed for fast recovery. This second requirement is met if all the
records in the logical log file are associated with closed transactions. If both
ofthese conditions are met, the logical log file is marked with status free and
it can be overwritten with new logical log records.
During processing, OnLine fills free logical log files in numeric sequence.
Whenthe first logical log file becomes full, OnLine begins to fill the next free
logfile. If the status of the next log file in the sequence is used insteadoffree,
normalOnLine processing is suspended. OnLine cannot skip the used log file
and begin filling some other, free log file. It is the OnLine administrator’s
responsibilityto ensure that free logical log files are always available during
processing.
OnLine requires a minimum of three logs to facilitate the rotation of the
logicallog files. While one log file receives the current records, OnLine might
bebacking up another log to tape. The third log is needed in case the current
log fills before the backup is complete. (This is similar to the strategy that is
used with the three logical log buffers.) (Refer topage 3-27 for more infor-
mation about logical log ID numbers and logical log file numeric sequence.
Refer topage 3-39 for more information about how to free a logical log file.)
Thelogical log backup tape is labeled with the unique number of the logical
logit contains. The logical log ID numbers increment each time a log is filled.
Forexample, in an OnLine configuration that contains three logical logs, the
log files receive the identification numbers 1, 2, and 3. The first time that
logical log file 1 is freed for reuse, it becomes logical log 4. The second time,
it will become logical log file 7. (For further information about logical log
identification numbers and logical log backup, refer topage 3-27.)