msc_iter_bdesc
if (msc_block_wrapped(msc_iter_bdesc(iter)))
if (!iter->wrap_count && msc_block_last_written(msc_iter_bdesc(iter)))
unsigned long data_bytes = msc_data_sz(msc_iter_bdesc(iter));
void *src = (void *)msc_iter_bdesc(iter) + MSC_BDESC;