IBM 000-8697 Manual do Utilizador descarregar pdf (Página 445)

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101

How to Improve Performance 5-5

Optimize Blobspace Blobpage Size

Familiarize yourself with the OnLine approach to blobspace blob storage

before you begin this section. Refer to page 2-148 and page 2-149 for

background information.

When you are evaluating blobspace storage strategy, you can measure

efﬁciency by two criteria:

■ Blobpage fullness

■ Blobpages required per blob

Blobpage fullness refers to the amount of data within each blobpage. Blobs

stored in a blobspace cannot share blobpages. Therefore, if a single blob

requires only 20 percent of a blobpage, the remaining 80 percent of the page

isunavailableforuse.However,youwanttoavoidmakingtheblobpagestoo

small. When several blobpages are needed to store each blob, you can

increase the overhead cost of storage. For example, more locks are required

for updates since a lock must be acquired for each blobpage.

tbcheck -pB and tbcheck -pe Utility Commands

To help you determine theoptimalblobpagesizefor each blobspace, use two

OnLine utility commands: tbcheck -pB and tbcheck -pe.

The tbcheck -pB command lists the following statistics for each table (or

database):

■ The number of blobpages used by the table (or database) in each

blobspace

■ The average fullness of the blobpages used by each blob stored as

part of the table (or database)

The tbcheck -pe command can provide background information about the

blobs stored in a blobspace:

■ Complete ownership information (displayed as database:owner.table)

for each table that has data stored in the blobspace chunk.

■ The number of OnLine pages used by each table to store its

associated blob data.

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