-groupdelay vs -Mf

[KlausE]

There are these two DB parameters: -groupdelay and -Mf

Group Delay (-groupdelay): 0-1000 ms, default is 1 for multi user
The number of milliseconds a transaction waits before committing. Use Group Delay (-groupdelay) to increase performance when Delayed BI File Write (-Mf) is set to zero.

Delayed BI File Write (-Mf): 0-32000sec, default 3 for multi user
Value in seconds of the delay before the database engine synchronously writes to disk the last before-image file records at the end of each transaction. It also specifies the interval that the broker process wakes up to make sure all BI file changes have been written to disk. The default is 3 for single-user batch jobs and for multi-user databases. Otherwise, the default is zero (0).

These parameters sound very similar to me. As -Mf is disabling -groupdelay by default, I assume -Mf is the newer and better one. -Mf is also seconds, not milliseconds so it can have a bigger impact.

But one customers says, that he has better single batch job performance with -Mf 0 and -groupdelay 1.

Can you tell me which is better or what is the difference in the mechanism?

thanks, Klaus

 

[RealHeavyDude]

The two parameters are not good for the same but they influence each other.

-Mf controls the delay until the before image buffers in the shared memory of the database broker are written back to disk asynchronously. If you set it to zero you basically write the modified before image buffers to disk synchronously. It is a compromise between performance on one hand and potentially losing committed transactions not written back to the before image file on the other hand. Increasing the setting will improve performance at the cost of potentially losing more data whereas decreasing the value will decrease potentially losing data at the cost of hurting performance.

With -groupdelay you can define a delay until a transaction is committed and thus the delay until a before image buffer is needed to write the transaction notes to, nevertheless the before image buffers are written to disk according to the -Mf setting.

Heavy Regards, RealHeavyDude.

 

[TomBascom]

Generally changing either of them is not useful.

If something thinks that it matters I would want to look very closely at their evidence before I jump to too many conclusions.