Wipe Free Space is time-consuming process. On modern large disks with terabytes of free space it may take several days to complete. Reserved Space functionality of BCWipe is intended to save the time by preserving the wiped space in clean state. As soon as Reserved Space is created, next time when Wipe Free Space process is running, BCWipe will recognize the clean space and won't wipe it.
Wipe Free Space process allocates free disk space temporarily and overwrites it according to the wiping scheme. Then the process deletes the temporary files; as a result, the disk has the same amount of free space as before. When Reserved Space feature is enabled, BCWipe does not delete some temporary files. The wiped space is still considered as "occupied" and it won't be used, so it won't become "dirty" and it won't need wiping.
The size of free and reserved space is maintained by BCWipe automatically. The user just assigns a threshold - the size of free space that remains after wiping. BCWipe will ensure that the drive never has lower free space than the 'threshold'.
If the reserved space is assigned, every time 'Wipe Free Space' is started and the process is successfully completed, BCWipe sets the amount of free space to be equal to the assigned threshold. The rest of newly wiped space is added to the reserved space.
After second and subsequent wiping, when new portion of wiped space has been added to the reserved space, an old portion of the reserved space mignt be freed. This is done to make all parts of disk work, to avoid using and reserving the same disk sectors for a long time.
BCWipe continuously monitors the amount of free space on the disk where Reserved Space feature is enabled. If the system or an application creates new file so that the actual free space becomes lower than the threshold, BCWipe automatically increases size of free space up to the threshold by reducing the reserved portion. If the reserved portion on the disk becomes empty, then BCWipe stops monitoring free space on the disk.