IBM 000-8697 Manual de usuario descargar pdf (Pagina 159)

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System Architecture 2-65

Physical Log Buffer

The contents ofthephysical log buffermust alwaysbeﬂushed to diskbefore

anydatabuffers.Thisruleisrequiredforfastrecovery.(Refertopage 4-39for

a deﬁnition of fast recovery. Refer to page 2-74 for a description of physical

log buffer ﬂushing when it is prompted by the need to ﬂush the shared-

memory buffer pool. Refer to page 2-72 for a description of the checkpoint

procedure.)

Flushing a Full Buffer

Buffer ﬂushing that results from the physical log buffer becoming full

proceeds as follows.

When a user process needs to write a page to the physical log buffer, it

acquires the latch associated with the physical log buffer and the latch

associated withthe physicallog ondisk. Ifanother userprocess is writing to

the buffer, theincoming userprocess must wait for the latches to be released.

After the incoming user process acquires the latches, before the write, the

user process ﬁrst checks the physical log for fullness. If the log is more than

75 percent full, the user process sets a ﬂag to request a checkpoint, performs

the write to the physical log buffer, and then releases the latch. The check-

point cannot begin until all shared-memory latches, including this one, are

released.

If the log is less than 75 percent full, the user process compares the incre-

mented pagecounter inthe physicallog buffer header to the buffercapacity.

If this one-page write does not ﬁll the physical log buffer, the user process

reserves space in thelog buffer for the write and releases the latch. Any user

process waiting to write to the buffer is awakened. At this point, after the

latch is released,the userprocesswritesthe page tothe reserved space inthe

physical log buffer. This sequence of events eliminates the need to hold the

latch during the write and increases concurrency.

If this one-page write ﬁlls the physical log buffer, ﬂushing is initiated as

follows.

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