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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
logically consistent. The physical log “before-images” can be combined with
the logical 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 a check-
point. The physical log is used in the first phase of fast recovery when OnLine
returns the 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 to page 4-39. For further information about the checkpoint procedure,
refer to page 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 to page 1-47.
The location of the physical log is specified in the configuration file
parameter PHYSDBS. 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.
The size of the physical log is specified, in kilobytes, in the configuration file
parameter PHYSFILE.
For further information about changing the physical log location and size,
refer to page 3-107.
The physical log is a set of contiguous disk pages, each of which contains a
copy of a specific OnLine page. The OnLine pages in the physical log can be
any OnLine page except a blobspace blobpage. Even overhead pages such as
chunk free-list pages, blobspace free-map pages, and blobspace bit-map
pages to the free-map pages are all copied to the physical log before data on
the page is modified and flushed to disk.