IBM 000-8697 Uživatelský manuál stáhnout pdf (Strana 138)

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2-44 IBM Informix OnLine Database Server Administrator’s Guide

Managing Shared-Memory Resources

OnLine resolves thedilemma by forcing anabort. When OnLine comes back

online, fast recovery occurs automatically. Fast recovery returns OnLineto a

consistent state through the last completed transaction.

(Refertopage 2-32forinstructionsontheproperwaytokilladatabaseserver

process. Refer to page 4-39 for further information about fast recovery.)

OnLine Timestamps

OnLine uses a timestamp to identify a time when an event occurred relative

to other events of the same kind. The timestamp is a 4-byte integer that is

assigned sequentially. The timestamp is not a literal time that refers to a

speciﬁc hour, minute, or second. When two timestamps are compared, the

one with the lower value is determined to be the older.

Each disk page has one timestamp in the page header and a second

timestamp in the last four bytes on the page. The page-header and page-

ending timestamps are synchronized after each write, so they should be

identical when the page is read from disk. Each read compares the times-

tamps as atestfor dataconsistency.If the testfails,an erroris returned to the

OnLine user process, indicating either that the disk page was not fully

written to disk, or that the page has been partially overwritten on disk or in

sharedmemory.(Refertopage 4-6for further information aboutconsistency-

checking errors and corrective actions.)

Refer to page 2-120 for further information about the layout of timestamp

information on a disk page.

In addition to the page-header and page-ending timestamp pair, each disk

page that containsablob also contains onememberof a secondpair of times-

tamps.Thissecondpairoftimestampsisreferredtoastheblobtimestamppair.

Theblobtimestampthatappears onthediskpageispairedwithatimestamp

thatisstoredwiththeforwardpointertothisblobsegment,eitherinthedata

row (with the blob descriptor) or with the previous segment of blob data.

(Refer to page 2-143for moreinformation aboutblob storage inthe datarow

and the blob descriptor.)

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