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 ALLBASE/SQL Performance and Monitoring Guidelines: HP 3000 MPE/iX Computer Systems > Chapter 1 Basic Concepts in ALLBASE/SQL Performance

Log Buffering
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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

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.



Log File Organization
  		 


Locking and Latching  
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