Function batch_span 

pub fn batch_span(offset: u64, body_len: usize, pipelined: bool) -> u64

How far the next batch starts from a batch of body_len bytes placed at offset (a multiple of 8): the body rounded up to 8 bytes, or, when pipelined, up to the end of the block it ends in, so that the next batch starts on a block boundary even when this one did not. Writes in flight together then never share a block, which O_DIRECT (whole-block writes) needs. A batch that would grow past MAX_BATCH that way keeps the 8-byte rounding.