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# DAH Performance

The DAH distributes actions between child servers. It also performs some additional processing on actions. For example, in mirror mode it can apply templates, and recognize and distribute administrative actions to all child servers.

Depending on your system, you can use DAH modes to reduce the amount of additional processing that DAH performs, which can improve the performance of the system.

## Fast Mirror Mode

In fast mirror mode, DAH distributes actions between mirrored child servers but does not perform any additional processing to actions. You can use this option to increase the performance of your IDOL system in mirror mode when you use DAH only for requesting information from child servers.

When running the DAH in fast mirror mode:

- You cannot send state-changing actions to the DAH (for example, actions including the `Delete`, `State` and `StoreState` action parameters).

- You cannot request additional processing on actions (for example, actions that use the `Template`, `EncryptResponse` or `Output` action parameters).

- You cannot use document IDs in action parameters.

- You cannot use distributed actions (actions to send to all child servers) or asynchronous actions.

## Simple Combinator Mode

In simple combinator mode, DAH does not use virtual databases to distribute and combine actions in non-mirror mode. DAH forwards the `DatabaseMatch` action parameter to child servers, and combines results from all child servers.

This option reduces the amount of processing that DAH must perform to distribute and combine virtual database.

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