IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 413)

100

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Data Consistency, Recovery, and Migration 4-41

Step 1: Checkpoint Condition

The ﬁrst step, returning all disk pages to their condition at the time of the

most-recent checkpoint, is accomplished by writing the “before-images”

stored in the physical log back to disk. Each “before-image” in the physical

log contains the address of a page that was updated afterthe checkpoint. By

writingeach“before-image”page in thephysicallogbacktodisk,changesto

OnLine data since the time of the most-recent checkpoint are undone.

Figure 4-4illustrates this step.(For moreinformation aboutthe contents and

function of the physical log, refer to page 2-152.)

Step 2: Find Checkpoint Record in Logical Log

The second step is tolocate theaddressof themost-recentcheckpoint record

in the logical log. The most-recent checkpoint record is guaranteed to be in

the logical log on disk.

All address information needed to locate the most-recent checkpoint record

in the logical log is contained in the active PAGE_CKPT page of the root

dbspace reserved pages.

Figure 4-4

Fast recovery,

step 1

Disk B

Fast Recovery: Step 1

Write “before-images”from the physical logback to disk,returning the datato

its state as of the most-recent checkpoint.

Disk A

Tblspace

Physical log

1 2 ... 408 409 410 411 412 413 414 415 416 417 418 ... 701 702

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