IBM 000-8697 Manual do Utilizador descarregar pdf (Página 169)

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System Architecture 2-75

Write Types Describe Flushing Activity

Ifthetimestamponthepageinthebufferpoolisequaltoormorerecentthan

the timestamp for the physical log buffer ﬂush, the “before-image” of this

page conceivably could be contained in the physical log buffer. If this is the

case, the physical log buffer must be ﬂushed before the shared-memory

buffer pages are ﬂushed.

Beforethe tbinit daemoncanﬂushthephysicallogbuffer, itmustacquirethe

latch for the shared-memory pointer structure and reset thepointer from the

current physical log buffer to the other buffer. After the pointer is reset, the

physical log buffer is ﬂushed.

Next, the daemon process updates the timestamp in shared memory that

describes the most-recent physical log buffer ﬂush. The speciﬁc page in the

shared-memory buffer pool that is marked for ﬂushing is ﬂushed. The

number of modiﬁed buffers in the queue is compared to the value of

LRU_MIN_DIRTY. If the number of modiﬁed buffers is greater than the value

represented by LRU_MIN_DIRTY,another pagebuffer is marked for ﬂushing.

The timestamp comparisonis repeated. If required,the physical logbufferis

ﬂushed again.

When no more buffer ﬂushing is required, the page-cleaner processes

calculate a value that represents a span of time during which the cleaner

processesremainasleep.This value, referredtoassnoozetime,is based on the

number of pages that the cleaner processes ﬂushed in this last active period.

(Refer to page 5-17 for further information about tuning the page-cleaning

parameters to improve OnLine performance.)

Write Types Describe Flushing Activity

OnLine provides you with some information about the speciﬁc condition

thatpromptedbuffer-ﬂushingactivity bydeﬁningsixtypesofOnLinewrites

and keeping count of how often each write occurs:

■ Sorted write

■ Idle write

■ Foreground write

■ LRU write

■ Chunk write

■ Big-buffer write

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