IBM 000-8697 Manual Do Utilizador

IBM Informix OnLine Database Server Administrator’s Guide

First, the page is written to the current physical log buffer, filling it. Next, the
user process latches the other physical log buffer. The user process switches
the shared-memory current-buffer pointer, making the newly latched buffer
the current buffer. The latch on the physical log on disk and the latch on this
new, current buffer are released, which permits other user processes to begin
writing to the new current buffer. Last, the full buffer is flushed to disk and
the latch on the buffer is released.

OnLine uses the shared-memory logical log buffer as temporary storage of
records that describe modifications to OnLine pages. From the logical log
buffer, these records of changes are written to the current logical log file on
disk, and eventually to the logical log backup tapes. Refer to

for a

description of the functions of the logical log files and their contents.

There are three logical log buffers. Each buffer is the size (expressed in
kilobytes) that is specified by the configuration file parameter

LOGBUFF

. This

triple buffering permits user processes to write to the active buffer while one
of the other buffers is being flushed to disk. Flushing might not complete by
the time the active buffer fills. Writing then begins in the third buffer. A
shared-memory pointer indicates the current buffer.