There are two strategies for inserting a filter device driver into the call-down path for a given unit's device support:
In most cases, the first strategy, in which the caller does not permanently allocate the unit, is simpler than the second. The filter device driver simply daisy-chains a filter indicator into the UNITINFO structure of the target unit; then, I/O that otherwise would go directly to the target unit's adapter device driver is redirected through the filter device driver.
The second strategy is required when the filter device driver needs to hide units. For example, a data-stripping feature can be implemented using a filter device driver as follows. The data-stripping filter device driver must allocate all target units to hide them from upstream device managers. Then the data-stripping filter device driver constructs a new UNITINFO table to contain the appropriate information for presenting a logical view of a single, logical (stripped) drive.