IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 158)

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2-64 IBM Informix OnLine Database Server Administrator’s Guide

Physical Log Buffer

Refer to page 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(inkilobytes)by the conﬁguration ﬁle parameterPHYSBUFF. Double

buffering permits user processes to write to the active physical log buffer

while the other buffer is being ﬂushed to the physical log on disk. A pointer

in shared memory indicatesthe current buffer to whichprocesseswrite 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.

Figure 2-8

The physical log

buffer and its

relation to the

physical log on disk

Current physical log

buffer (now ﬁlling)

Physical log buffer

(ﬂushing)

Writesperformed byOnLine

user processes

Writes performed by

page-cleaner daemons

Physical log buffers

Physical log ﬁles

1 2 ... 153 154 155 156 157 158 159 160 161 162 163 ... 701 702

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