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Database Administration
Maintaining and Monitoring Your Database : Managing Performance : Server performance factors : Before-image I/O : Setting a BI threshold
Setting a BI threshold
When an application performs large schema updates or large transactions, the BI clusters can grow in excess of 2GB. If a crash occurs during such an operation, the recovery process might require several times the amount of disk space as the BI log was using at the time of the crash. Often this space is not available, leaving the database in an unusable state.
Using the Recovery Log Threshold (-bithold) startup parameter sets the maximum size to which BI files can grow. Once the threshold is reached, the database performs an emergency shutdown. This mechanism ensures that there will be enough disk space to perform database recovery. All messages associated with the threshold are logged in the database log (.lg) file. These messages include:
*Value of the threshold
*Warning message if the threshold is set above 1000MB
*Warning message when recovery log files are extended
*Message that a database shutdown is occurring because the threshold has been reached
The recommended range is to set -bithold between three and one hundred percent (3-100%) of the largest possible recovery log file size, rounded to the nearest cluster boundary. If the threshold is set above 1000MB, the database engine issues a warning message to the display and the database log (.lg) file. The system will check the total amount of BI clusters in use each time a new cluster is marked as used. If the No Crash Protection (-i) is set, the recovery log threshold parameter is set to the default (none) and cannot be overridden.