The present invention provides a method for the allocation of data on physical media by a file system which optimizes power consumption. The present invention improves power efficiency of computing devices using file systems by allocating data in a way that allows devices to power down when they are not in use. This is carried out by using an allocation group method to divide the storage system into groups of one or more disk drives. The allocator fills the first group entirely before moving to the second group. The allocator globally allocates blocks within each group. Groups are typically sized to meet the average I/O bandwidth requirement of the application. The method also dynamically increases the number of active allocation groups based on I/O load. This is done by computing the average number of write requests per time period.