IBM 000-8697 Manual do Utilizador descarregar pdf (Página 455)

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How to Improve Performance 5-15

How Is Tuning Done?

You can increase the number of shared-memory resources by increasing the

valueofTBLSPACES,LOCKS,orUSERS intheconﬁgurationﬁle.Ifyouincrease

thevalue of USERS,youmight need toalsoincreasethevalue of TBLSPACES,

LOCKS, and BUFFERS, since theminimum valuesfor all threeof these param-

eters are based on the value of USERS. For further information about how to

change the value of these parameters, refer to the following pages:

TBLSPACES, page 3-113; LOCKS, page 3-112; and USERS, page 3-114.

Log Buffer Size

The optimal size for the physical and logical log buffers depends on your

environment.In general, thelog buffersshould belargeenough to minimize

physical I/O writing to the logs on disk. However, the buffers should not be

so large that you have allocated shared-memory space that could be used

more efﬁciently for some other purpose.

Asecond consideration istheamountofdata that isheldinvolatile memory.

This is a concern only if you are using buffered logging. The larger the log

buffer, the more log data that can be lost in the event of operating system

failure.Logdatathat is lostcannotbeused during fastrecovery.Therefore,if

several COMMIT records are left in the logical log buffer (database uses

buffered logging) and lost, you cannot recover these transactions after a

failure. Thus, if any of your OnLine databases use buffered logging, you

should weigh the beneﬁts of increased buffer size against the disadvantages

of possible data loss in the event of operating system failure.

(The following paragraphs rely on information presented on page 5-11 that

explains the Bufsize and Pages/IO ﬁelds andhow to interpret theirvalues.)

Logging Status

An additional consideration in the decision to resize the logical log buffer is

the complication of the database logging status. The logging status of the

database affects the logical log Pages/IO value. If a database uses unbuf-

fered logging, the Pages/IO value is close to 1. If a database uses buffered

logging, the Pages/IO value should be very close to the value of Bufsize.

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