Second, the numbers might not add up exactly as you wouldexpect them to. In particular, operations across RAID-Z and mirrored deviceswill not be exactly equal. This difference is particularly noticeable immediatelyafter a pool is created, as a significant amount of I/O is done directly tothe disks as part of pool creation, which is not accounted for at the mirrorlevel. Over time, these numbers gradually equalize. However, broken, unresponsive,or offline devices can affect this symmetry as well.