IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 171)

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System Architecture 2-77

Write Types Describe Flushing Activity

If OnLineis conﬁgured for more thanone page-cleaner daemon process,the

LRU queues are divided among the page-cleaner daemons for more efﬁcient

ﬂushing.

Foreground Write

If a database server process searches through the FLRU queues and cannot

locate an empty or unmodiﬁed buffer, the server process itself marks a page

forﬂushing.Iftheserverprocessmustperformbufferﬂushingjusttoacquire

a shared-memory buffer, performance can suffer. Writes that the server

process performs are called foreground writes. Foreground writes should be

avoided. If you ﬁnd that foreground writes are occurring, increase the

number ofpage cleaners or decreasethe value of

LRU_MAX_DIRTY. (Refer to

page 5-17 for more information about tuning the values of the page-cleaner

parameters.)

LRU Write

Foreground writes alert the master daemon, tbinit, that page cleaning is

needed. Once alerted, the tbinit daemon wakes the page cleaners or, if none

have been allocated in this OnLine conﬁguration, the tbinit daemon begins

pagecleaning. An LRU writeoccursasa resultof tbinit prompting,insteadof

as a result of the page cleaners waking by themselves.

Chunk Write

Chunk writes are performed by page cleaners during a checkpoint or when

every page in the shared-memory buffer pool is modiﬁed. Chunk writes,

which are performed as sorted writes, are the most efﬁcient writes available

to OnLine.

During a chunk write, each page cleaner is assigned to one or more chunks.

Each page cleaner reads through the buffer headers and creates an array of

pointers to pages that are associated with its speciﬁc chunk. (The page

cleaners have access to this information because the chunk number is

containedwithinthephysicalpagenumberaddress,whichispartofthepage

header.) This sortingminimizes headmovement (disk seektime) on thedisk

and enables the page cleaners to use the big buffers during the write, if

possible. (Refer to page 2-55.)

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