A primary physical storage device has effectively limitless apparent free space. Responsive to receiving a request to dynamically allocate an amount of desired free space on the primary device to store new data on the primary device, and responsive to determining that an amount of actual free space on the primary device is insufficient to permit such allocation, existing data stored on the primary device is moved to a secondary storage device. The first existing data appears to still be stored on the primary device. Responsive to receiving a request to retrieve existing data from the primary device, and to determining that the existing data has been moved to the secondary device, the existing data is moved back to the primary device. The existing data was originally stored on the primary device, and is currently stored on the primary device or has been moved to the secondary device.