IBM 000-8697 Manual de usuario descargar pdf (Pagina 412)

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4-40 IBM Informix OnLine Database Server Administrator’s Guide

Fast Recovery and Logging

The aim of fast recovery is to return OnLine to a consistent state as part of

shared-memoryinitialization.TheactionsthatOnLinetakesasitimplements

fast recovery can be summarized in four steps:

1. Returnalldiskpagestotheirconditionat the time of the most-recent

checkpoint using the data in the physical log. (See Figure 4-4 on

page 4-41.)

2. Locate the most-recent checkpoint recordin the logicallog ﬁles. (See

Figure 4-5 on page 4-42.)

3. Roll forward all logical log records written after the most-recent

checkpoint record. (See Figure 4-6 on page 4-43.)

4. Roll back transactions that do not have an associated COMMIT

(commit work) record. (See Figure 4-7 on page 4-44.)

The result is that OnLine data isreturned to a consistent state: all committed

transactions are restored and all uncommitted transactions are rolled back.

Fast Recovery and Logging

If adatabase uses buffered logging, some logical logrecords associated with

committed transactions might not be written to the logical log at the time of

thefailure.If this occurs, fastrecoveryisunableto restorethosetransactions.

Fast recovery can only restore transactions with an associated COMMIT

(commit work) record stored in the logical log or on disk. (This is why

buffered logging represents a trade-off between performance and data

vulnerability.)

For databases that do not use logging, fast recovery restores the database to

its state at the time of the most-recent checkpoint. All changes made to the

database since the last checkpoint are lost.

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