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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.
A record of the blobspace operation is written to the logical log buffer, but the
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-
map pages and the bit-map pages. (Refer to page 2-148 for information about
the blobspace overhead pages.) By logging 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
process must somehow locate and copy to the logical log tape each 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 written to a blobspace, it is the function of the blobspace free-
map page to allocate and track the blobpages that were used to store the blob
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. The entry also includes the logical log ID number that was currentwhen
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.