IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 168)

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2-74 IBM Informix OnLine Database Server Administrator’s Guide

How OnLine Synchronizes Buffer Flushing

Bufferﬂushingoccurs within thecontextofOnLine activity. When OnLineis

ﬁrstinitiated,allbuffersareempty. Asprocessingoccurs,datapagesareread

from disk into the buffers and user processes begin to modify these pages.

(Refertopage 2-73foranexplanationofthe“before-imagesﬁrst”rule,which

isthereasonthatsynchronizationisnecessary.In addition,page 2-73liststhe

four events that prompt buffer-pool ﬂushing and cross-references to further

background information.)

Before a page in shared memory is modiﬁed for the ﬁrst time, a copy of the

page“before-image”is written tothephysical log buffer. Subsequentmodiﬁ-

cations to that page in shared memory do not result in additional “before-

images”being written to thephysicallog;onlytheﬁrst modiﬁcation does so.

After each modiﬁcation, a record of the change is written to the logical log

buffer if the database was created with logging or if the change affected the

database schema.

MLRU queuesbegintoﬁllwithmodiﬁedpages. Each modiﬁedpageincludes

a timestamp that describes the time at which the page was modiﬁed.

Eventually the number of modiﬁed buffers in the MLRU queues reaches

LRU_MAX_DIRTY and page cleaning begins.

The pages in the physical log buffer are always ﬂushed to disk prior to

ﬂushing the pages that are contained in the modiﬁed buffers in the shared-

memory buffer pool. (Refer to page 2-73.)

When page cleaning is initiated from the shared-memory buffer pool, the

daemon process performing the page cleaning must coordinate the ﬂushing

so that the physical log buffer is ﬂushed ﬁrst.

Howisthisdone? The answer istimestampcomparison.(Refer to page 2-44 for

a deﬁnition of a timestamp.)

Shared memory contains a structure that stores a timestamp each time the

physical log buffer is ﬂushed. If a tbpgcl or tbinit daemon needs to ﬂush a

page in a shared-memory buffer, the daemon process compares the

timestampinthemodiﬁedbufferwiththetimestampthatindicatesthepoint

when the physical log buffer was last ﬂushed.

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