IBM 000-8697 Manuel d'utilisateur télécharger pdf (Page 396)

100

101

4-24 IBM Informix OnLine Database Server Administrator’s Guide

Blobspace Logging

Operations Logging

OnLine does not copy blobspace data to the logical log ﬁles. This is the

important difference between the way that OnLine logs blobspace data and

dbspace data.

Arecordof theblobspaceoperationiswrittentothelogicallogbuffer,butthe

logical log records do not include a copy of the blobspace data. The logical

log records only include images of the blobspace overhead pages: the free-

mappagesandthebit-mappages. (Refer topage 2-148forinformationabout

the blobspace overheadpages.) Bylogging these overhead pages,the logical

log ﬁle records track blobpage allocation and deallocation (when blobs are

deleted from blobpages).

Since the logical log records do not include blobspace blob data, the tbtape

processmust somehow locateandcopy tothelogical log tapeeach blobpage

that was allocated while this logical log ﬁle was current.

How does tbtape know which blobpages to copy? It reads the information

from the entries in the blobspace free-map page.

When a blob is writtento a blobspace, it is the function of the blobspace free-

mappageto allocate and tracktheblobpagesthatwereusedtostoretheblob

data. Therefore, as part of blobpage allocation, an entry is placed in the free-

map page indicating the blobpages that are now allocated and contain blob

data.TheentryalsoincludesthelogicallogIDnumberthatwascurrentwhen

the blobpage was allocated. By reading this information, the tbtape process

can identify all blobpages that are associated with the insert, update, and

delete records contained in a speciﬁc logical log ﬁle. (Refer to page 4-26.)

Operations Rollback

How can a blobspace operation be rolled back if the logical log ﬁle does not

contain a copy of the data that was originally inserted?

The answer is that OnLine always has access to a copy of the blobspace data

until the transaction is committed. However, the accessible copy of the data

is maintained either in the blobspace on disk or on the logical log backup

tape, not in the logical log itself. The logical log is needed, because the log

contains the blobspace control pages, which track the location and status of

the blobpages.

1 2 ... 391 392 393 394 395 396 397 398 399 400 401 ... 701 702

Commentaires sur ces manuels

Pas de commentaire

IBM 000-8697 Manuel d'utilisateur Page 396

Commentaires sur ces manuels

Produits connexes et manuels pour Logiciel de base de données IBM 000-8697