IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 215)

100

101

System Architecture 2-121

Structure of a Dbspace Page

Page Header

The page header includes six components:

■ Page identiﬁcation number (address of the page on disk)

■ Number of slot table entries used on the page (used to calculate

where to locate the next slot table entry)

■ Number of free bytes left on the page

■ Pointertothecontiguousfreespaceonthepagethatliesbetweenthe

last data entry and the ﬁrst slot table entry

■ Timestamp that changes each time the page contents are modiﬁed

■ Two index-related pointers (used if the page is used as an index

page)

Timestamp

The page-header timestamp and the page-ending timestamp function as a pair to

validate page consistency. Each time the page contents are modiﬁed, a

timestamp is placed in the page header. At the end of the write, the header

timestampiscopiedintothe lastfourbytesonthepage.Subsequentaccess to

the page checks both timestamps. If the two timestamps differ, this inconsis-

tency is reported as a part of consistency checking. (Refer to page 4-6 for

further information about consistency checking errors and corrective

actions.)

Slot Table

The slot table is a string of 4-byte slot table entries that begins at the page-

ending timestamp and grows toward the beginning of the page. The entries

in the slot table enable OnLine user processes to ﬁnd data ondbspace pages.

Each entry intheslot table describesonesegment ofdatathat is storedin the

page.Thenumberoftheslottableentryisstoredasa1-byteunsignedinteger.

Theslottableentriescannotexceed255.Thisistheupperlimitonthenumber

of rows, or parts of a row, than can be stored in a single data page.

The slot table entry is composed of two parts:

■ Page offset where the data segment begins

■ Length of the data segment

1 2 ... 210 211 212 213 214 215 216 217 218 219 220 ... 701 702

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