2-152 IBM Informix OnLine Database Server Administrator’s Guide
Physical Log
Physical Log
The function of the physical log is to maintain a set of “before-images” of
dbspace pages that represent a time at which all data is both physically and
logicallyconsistent. The physical log “before-images” can be combined with
thelogical log records of transactions to recover all transactions that occurred
since the most-recent point of known consistency. The point of known
physical consistency in an OnLine database server system is called acheck-
point.The physical log is used in the first phase of fast recovery when OnLine
returnsthe entire system to the state of the most-recent checkpoint (the point
of known physical consistency).
For further information about the role of the physical log in fast recovery,
refer topage 4-39. For further information about the checkpoint procedure,
refer topage 2-72.
When OnLine is initialized, the physical log is created in the root dbspace.
After OnLine has been taken to quiescent mode, you can move the physical
log to another dbspace. You may want to do this to try to improve perfor-
mance. Refer topage 1-47.
The location of the physical log is specified in the configuration file
parameterPHYSDBS. This parameter should be changed only if you decide to
move the physical log file from the root dbspace. Otherwise, the parameter
contains the name of the root dbspace by default.
Thesize of the physical log is specified, in kilobytes, in the configuration file
parameterPHYSFILE .
For further information about changing the physical log location and size,
refer topage 3-107.
The physical log is a set of contiguous disk pages, each of which contains a
copyof a specific OnLine page. The OnLine pages in the physical log can be
anyOnLine page except a blobspace blobpage. Even overhead pages such as
chunk free-list pages, blobspace free-map pages, and blobspace bit-map
pagesto the free-map pages are all copied to the physical log before data on
the page is modified and flushed to disk.