IBM 000-8697 Manual de usuario descargar pdf (Pagina 167)

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System Architecture 2-73

When the Daemons Flush the Buffer Pool

Buffer ﬂushing is managed by the tbinit master daemon and performed by

tbinit or by one or more tbpgcl (page-cleaner) daemons. (If no tbpgcl

daemons have been conﬁgured for your OnLine server, the tbinit daemon

performs page-cleaner functions.)

Flushing the modiﬁed shared-memory page buffers, the physical log buffer,

and the logical log buffer must be synchronized with page-cleaner activity

according to speciﬁc rules designed to maintain data consistency.

The overriding rule of buffer ﬂushing is this: ﬁrst ﬂush the “before-images”

of modiﬁed pages to disk before you ﬂush the modiﬁed pages themselves.

Inpractice,thismeansthattheﬁrstphysicallogbufferisﬂushedandthenthe

buffers containing modiﬁed pages from the shared-memory buffer pool.

Therefore, even when the need to ﬂush a shared-memory page buffer arises

because that buffer is needed by another user process (a foreground write,

refer to page 2-75), the page buffer cannot be ﬂushed until it is veriﬁed that

the“before-image”ofthepagehasalreadybeenwrittentodisk.Ifthiscannot

be veriﬁed, the physical log buffer must be ﬂushed ﬁrst, before the single

shared-memory page buffer is ﬂushed. (Refer to page 2-74 for more infor-

mation about how this sequence of events is enforced.)

Buffer-pool ﬂushing is initiated under any one of four conditions:

■ A requirement for page cleaning, determined by the value of

LRU_MAX_DIRTY (refer to page 2-58)

■ A need to ﬂush a fulllogical logbufferorphysicallogbuffer(referto

page 2-63 and page 2-66, respectively)

■ Aneedtoﬂushthelogicallogbufferafteracommittedtransaction in

an unbuffered database (refer to page 2-66)

■ A need to execute a checkpoint (refer to page 2-70)

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IBM 000-8697 Manual de usuario Pagina 167

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