IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 172)

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2-78 IBM Informix OnLine Database Server Administrator’s Guide

Writing Data to a Blobspace

In addition, since database server processes must wait for the checkpoint to

complete, thepage-cleaner daemonsare not competing witha large number

ofprocessesfor CPU time. As aresult, the pagecleaners canﬁnish their work

with less context switching.

Big-Buffer Write

Each OnLine big buffer is the size of eight regular buffers, or eight times

BUFFSIZE. Whenever multiple pages to be written to disk are physically

contiguous,OnLineusesabigbuffertowriteuptoeightpagesinasingle

I/O

operation. Refer to page 2-55 for more details about the big buffers.

Writing Data to a Blobspace

Blobdata(BYTE andTEXTdatatypes)iswrittentoblobspacepagesaccording

toa procedurethatdiffersgreatly fromthe single-page I/O that is performed

whenthe shared-memorybufferpoolis ﬂushed. (Blobdata that isstoredina

dbspace is written to disk pages in the same way as any other data type is

written. Refer also to page 2-148 for more information about the structure of

a blobspace blobpage and to page 2-145 for more information about the

structure of a dbspace blob page.)

OnLine provides blobspace blobpages to store large BYTE and TEXT data

types. OnLine does not create or access blobpages by way of the shared-

memorybufferpool.Blobspaceblobpagesarenotwrittentoeitherthelogical

or physical logs.

The reason that blobspace data is not written to shared memory or to the

OnLine logs is that the data is potentially voluminous. If blobspace data

passed throughthe shared-memory pool, itwoulddilute the effectivenessof

thepoolbydrivingoutindexanddatapages.Inaddition,themanykilobytes

of data per blobspace blob would overwhelm the space allocated for the

logical log ﬁles and the physical log.

Instead, blobpage data is written directly to disk when it is created.

Blobpages stored on magnetic media are written to archive and logical log

tapes, but not in the same method as dbspace pages. (Refer to page 4-22 for

further information about blobspace logging.)

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