If you get a comment that says "forward iteration/iterator is not done by block", it means that you used a list or a stack to keep all the log records (up to the last check point) in memory for the forward iteration (i.e. redo) phase. This is incorrect because the number of log records can be very large and may not even fit in memory. The correct implementation should keep no more than one block of log in memory at any time, like the backward iterator.