For operations involving a change to data, ALLBASE/SQL writes log records to
a log file, so that these operations can be rolled back if necessary
or reapplied in the event of a system failure.
In logging, the transaction enters log records into the log buffer,
which is periodically flushed (written to disk). The log buffer pool
consists of a number of 512-byte pages configured with the START DBE
statement or with the SQLUtil ALTDBE command.
Regardless of log type, the log buffer pool is used as the collection point
for log records. The maximum log buffer size is 1024 pages, but 120
pages is generally recommended. For detailed information about
creating and manipulating log files, refer to the "Backup and Recovery"
chapter in the ALLBASE/SQL Database Administration Guide.
To monitor logging I/O, or to check the maximum size of the log buffer,
run SQLMON and invoke the IO screen.
No-Log Pages |
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Some operations that change data in a DBEnvironment allocate new
pages for insertion in a DBEFile. These new pages are often called
no-log pages. The use of no-log pages provides a performance benefit
in non-archive mode. In non-archive mode, no-log pages are not logged;
only a log record is created for the allocation of the page. If
the operation must be rolled back, then the page can simply be
de-allocated. If a savepoint is defined following the allocation
of a no-log page, the page is marked for logging, since rollback of
a no-log page is an all-or-nothing operation; that is, the page
could be deallocated, but not partially unfilled.
Since the content of a no-log page is not logged, the page is
forced to disk at COMMIT WORK time.
No-log pages include the following:
Pages allocated at INSERT time in non-archive logging. When you are
inserting rows, your transaction obtains an exclusive lock on the
page being allocated, so other transactions cannot write on that page.
Thus it is possible to log only the allocation of the page, not the
data being inserted by your transaction.
Pages allocated at CREATE INDEX time during non-archive logging.
Pages allocated in a B-tree split during non-archive logging,
if your transaction has obtained
an exclusive lock on the table.
If you must insert a large number of rows into a table that
already has an index defined on it, B-tree index pages will split
as they begin to fill up. If your transaction has locked the
table in exclusive mode, it is only necessary to log the allocation
of new pages in non-archive mode. In the event of a rollback,
those pages can simply be deallocated.
This behavior of no-log pages makes it possible to save considerable
logging activity in non-archive mode for
LOAD and INSERT operations that lock the table in exclusive mode.
However, there may be an increase in I/O activity as pages are
forced to disk at COMMIT WORK time.
No-log pages are not created in archive mode. In archive mode, all changes to the DBEnvironment are logged.
