IBM 000-8697 Manual de usuario descargar pdf (Pagina 157)

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System Architecture 2-63

Physical Log Buffer

If the user process intends to modify the buffer, it acquires a latch on the

buffer and changes the buffer lock-access type to exclusive.

A copy of the “before-image”of the page is needed for data consistency. The

userprocessdeterminesifa“before-image”ofthispage waswrittentoeither

the physical log buffer or the physical log since the last checkpoint. If not, a

copy of the page is written to the physical log buffer.

The data in the page buffer is modiﬁed, including the timestamps on the

page. When the modiﬁcation is complete, the latch on the buffer is released.

If any transaction records are required for logging, those records are written

to the logical log buffer.

After the latch on the buffer is released, the user process is ready to release

thebuffer. First,thereleasinguserprocess acquiresa latch on thebuffertable

that enables it to modify the buffer entry.

The releasing process updatesthe timestampin thebuffer header so that the

timestamp on the buffer page and the timestamp in the header match.

Statistics describing the number and types of writes performed by this user

process are updated.

The lock is released as described in the previous section, but the buffer is

appendedtotheMLRU queueassociatedwithitsoriginalqueueset.(Referto

page 2-57). If the latch for that MLRU queue is unavailable, the process tries

to acquire a latch for a randomly selected MLRU queue. When the MLRU

queue latch is acquired, the modiﬁed buffer is linked to the “most-recently

used” end of the queue.

Physical Log Buffer

OnLine uses the shared-memory physical log buffer as temporary storage of

“before-images”ofdisk pages. Beforea disk pagecan be modiﬁed,a“before-

image”ofthe page ondiskmustalreadybestoredin the physicallogondisk

or one must be written to the physical log buffer. In the latter case, the

physical log buffer must be ﬂushed to disk before the modiﬁed page can be

ﬂushed to disk. Writing the “before-image” to the physical log buffer and

then ﬂushing the buffer page todisk isillustrated in Figure 2-8on page 2-64.

Both the physical log buffer and the physical log maintain the physical and

logical consistency of OnLine data.

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