Don't Make ZFS Re-Read What It Just Read

I have already mentioned one of my improvements to scrub in an earlier post, Teaching ZFS about time.
In that post I also claimed that scrub can be expensive, and that we were looking for ways to limit the amount of data we actually want to scrub.
A scrub on a multi-petabyte pool can run for days, and some of us want a smaller, easier check that everything is still going right.