1-26 IBM Informix OnLine Database Server Administrator’s Guide

Logical Log Configuration Guidelines

Referto page 1-42for guidelines on deciding how much disk space should be
dedicated to OnLine dbspaces. Refer to page1-26 for information about
sizing the logical log files.
The default value ofPHYSDBS is 1,000 KB.
The default values included in thetbconfig.std file adhere to both of the
guidelinesjust described. The size of the physical log is 1,000 KB. The default
value ofLOGSIZE is 500 KB. The default value of LOGFILES is 6. Thus, total
logical log size is 3,000 KB. Total space devoted to the physical and logical
logs is 4,000 KB. This value meets the first criterion of 20 percent of the root
dbspace, which is 20,000 KB. The strategy also meets the second recommen-
dationto allocate logging space in a ratio of 3:1, logical log space to physical
log space.
Logical Log Configuration Guidelines
Thissection describes how to assign initial configuration values to the logical log
parameters. Refer topage 3-14 for a detailed discussion of logical log configu-
rationguidelines. Refertopage 4-18 for anoverviewof themechanicsof OnLine
blobspace and dbspace logging.
OnLine supports transaction logging, which is the ability of the database
server to track and, if needed, to roll back all changes made to the database
duringapplication transactions. OnLine transaction logging is implemented
byrecording each change made to a database in disk space allocated for the
OnLine logical log files.
Thelogical log files contain a history of all database changes since the time of
the last archive. At any time, the combination of OnLine archive tapes plus
OnLine logical log files contain a complete copy of your OnLine data.
AsOnLine administrator, you decide on the optimum total size of the logical
log:LOGFILES multiplied by LOGSIZE. The optimum size of the logical logs is
based on the length of individual transactions. (OnLine does not permit a
singletransaction to span all logical log files.) Refer to page 2-156for detailed
information on selecting values for LOGFILES andLOGSIZE that are specifi-
cally tuned to your application environment.