IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 254)

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2-160 IBM Informix OnLine Database Server Administrator’s Guide

Long Transactions

The second long-transaction high-water mark, LTXEHWM, indicates that the

logical log has ﬁlled to a critical level. Most user processes are denied access

to the logical log. Only user processes currently rolling back transactions

(including the long transaction) and database server processes currently

writingCOMMIT recordsareallowed access to thelogicallog.Theintent is to

preserve as much space as possible for rollback records being written by the

user processes that are rolling back transactions.

If LTXHWM is deﬁned as 50, a long transaction condition exists when 50

percentof the logicallog space isconsidered“used.” The problem presented

by the long transaction is this: to increase the amount of free space in the

logical log, you must free one or more of the logical log ﬁles. However,

OnLine cannotfree a logical log ﬁle untilall thetransactions associated with

the records in the ﬁle are closed. If a single transaction stays open for an

extended period of time, OnLine cannot free the log ﬁle where that trans-

action began. Because OnLine writes to the logical log ﬁles in a sequential

order,if OnLine tries to write in the next log ﬁle and ﬁndsthatitis“used,”all

OnLine processing issuspended. OnLinecannot skip over a used logical log

ﬁle to ﬁnd another that is free.

When the logical log ﬁlls to the high-water mark speciﬁed by LTXHWM, the

tbinit daemon begins searching for an open transaction in the oldest, used

(but not freed)logical log ﬁle.Ifa long transactionis found, tbinitdirects the

executing databaseserver process tobegin to roll back thetransaction. More

than one transaction may be rolled back if more than one long transaction

exists.

The transaction rollback itself generates logical log records, however, and as

other processes continue writing to the logical log, the log continues to ﬁll.

Thegoalistofreetheoldestusedlogicallogﬁlebeforethelogﬁllstoacritical

point.

As the logical log continues to ﬁll, it might reach a second high-water mark

speciﬁed as the exclusive-access, long-transaction high-water mark. This second

boundary is speciﬁed by the LTXEHWM conﬁguration ﬁle parameter. The

default value of LTXEHWM is 60 percent.

If the logical log ﬁles ﬁll to the point deﬁned by LTXEHWM, most OnLine

server processes are denied access to the current logical log ﬁle. Only

database server processes that are rolling back transactions are allowed to

write to the ﬁle. (If a database server process is currently writing a COMMIT

record or is currently rolling back a transaction, it is allowed to continue.)

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