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Database Administration
Maintaining and Monitoring Your Database : Managing Performance : Server performance factors : Before-image I/O : Delaying BI writes
Delaying BI writes
When the Delayed BI File Write (-Mf) startup parameter is set to zero, use the Group Commit technique to increase performance. This technique assumes that for the benefit of overall performance, each individual transaction can take slightly longer. For example, when a transaction begins to commit and spools its end note to the BI buffer, it waits a short time until one of two things happen: it fills the buffer and is written to disk, or a few other transactions complete and store their end notes in the BI buffer so that a single synchronous write commits all the transactions. Use the Group Delay (-groupdelay) startup parameter to set the amount of time (milliseconds) the transaction waits.
If the Group Commit technique does not provide sufficient improvement, you can improve performance on a busy system by delaying BI file writes with the Delayed BI File Write (-Mf) startup parameter.
By default, the database engine writes the last BI block to disk at the end of each transaction. This write guarantees that the completed transaction is recorded permanently in the database. On a system with little update activity, this extra BI write is very important and adds no performance overhead. On a busy system, however, the BI write is less important (the BI block will be written to disk very soon anyway) and might incur a significant performance penalty.
Set the -Mf parameter to delay BI writes at transaction commit time. When -Mf is set to a positive value, the last BI record is guaranteed to be written to disk within the specified number of seconds. The record is written sooner if the user logs out or the system shuts down.
Note: Suppressing the last BI write does not reduce database integrity. However, if there is a system failure, the last few completed transactions can be lost (never actually written to the BI file).
For more detailed information on the -Mf parameter, see Database Startup Parameters.