Configuring the Size of the Physical Log

Because a checkpoint logically empties the physical log when it becomes 75 percent full, it is unlikely that the log would become 100 percent full before the checkpoint completes. To assure further that the physical log does not become full during a checkpoint, take the following actions:

• Configure the database server according to the sizing guidelines for the physical log and the logical-log files.
• Fine-tune the size of the physical log by monitoring it during production activity.

Fuzzy checkpoints keep the physical log from filling up too quickly when applications are doing intensive updates. (See Fuzzy Checkpoint.) However, the physical log could still become full, as the following sections describe.

Physical-Log Overflow When Many Users Are in Critical Sections

A checkpoint cannot occur if any thread is in a critical section. (See Critical Sections.) This scenario can occur when the thread that does the checkpoint also rolls back a long transaction.

Effect of Checkpoints on the Physical-Log Size

Fuzzy checkpoints keep the physical log from filling up too quickly when applications are doing intensive updates. You can reduce the size of the physical log when applications require less intensive updates or when updates tend to cluster within the same pages. You can decrease the size of the physical log if you intend to use physical-log fullness to trigger checkpoints.

If you increase the checkpoint interval or anticipate increased activity, consider increasing the size of the physical log. For more information, see the chapter on effects of configuration on I/O activity in your IBM Informix Dynamic Server Performance Guide.

Physical-Log Overflow When Transaction Logging Is Turned Off

The physical log can overflow if you use simple large objects or smart large objects in a database with transaction logging turned off, as the following example shows.

When the database server processes these simple large objects, each portion of the simple large object that the database server stores on disk can be logged separately, allowing the thread to exit the critical sections of code between each portion. However, if logging is turned off, the database server must carry out all operations on the simple large object in one critical section. If the simple large object is large and the physical log small, this scenario can cause the physical log to become full. If this situation occurs, the database server sends the following message to the message log:

Physical log file overflow

The database server then initiates a shutdown. For the suggested corrective action, refer to this message in your message log.