IBM 000-8697 Double Buffering, Causes of Flushing

2-64 IBM Informix OnLine Database Server Administrator’s Guide
Physical Log Buffer
Refer topage 2-151 for further information about the physical log.
Double Buffering
The physical log buffer is actually two buffers. The size of each buffer is
specified(in kilobytes) by the configuration file parameter PHYSBUFF.Double
buffering permits user processes to write to the active physical log buffer
whilethe other buffer is being flushed to the physical log on disk. A pointer
inshared memory indicates the current buffer to which processes write their
“before-images.”
Causes of Flushing
Three events cause the physical log buffer to flush:
■One of the physical log buffers becomes full.
■A modified page in shared memory must be flushed.
■A checkpoint occurs.
Figure2-8
The physical log
buffer and its
relation to the
physicallog on disk
Current physical log
buffer (now filling)
Physical log buffer
(flushing)
Writesperformed by OnLine
user processes
Writes performed by
page-cleaner daemons
Physical log buffers
Physical log files