This paper considers replacement polices for an extended 
 cumulative damage model with minimal maintenance at each shock 
 and minimal repair at failure: 
 A system is replaced at time $T$ or at shock number $N$, 
 and undergoes maintenance or repair between replacements. 
 The expected cost rate is obtained and each optimal $T^*$ 
 and $N^*$ to minimize the cost rate is discussed. 
 It is shown that this model would be applied to the backup 
 of secondary storage files in a database system as an example.