The present invention describes a warming procedure for warming redundant functional units. The solution described in the present invention keeps the functional unit during most of the warming in a state where it is able to serve new configuration requests. The working unit side may still be locked but for a shorter amount of time than earlier. The present invention makes it easy to warm various kinds of data areas in multiple clients without a need to organize or classify the data for warming purposes. Data is treated as bulk bytes.The present invention further describes a warming manager which controls the warming procedure. The warming procedure is based on the fact that the warming manager takes one or more snapshots of the dynamic data of the warmable objects, and transfers only differences in the dynamic data between two consecutive snapshots to the warming manager of the spare unit side.