A: The overhead is very small – usually in the range of .05% to .10% of the total storage capacity.  The overhead is 8 bytes per tape block.  Thus if you write a 10 Gbyte tape using a block size of 16K, the overhead is .05% and it shrinks to .02% if a 32K block size is used.  The larger the block size, the smaller the overhead.  But even writing 2K blocks, the overhead is only .4% in the above example!