* and sets the on-disk bit (lazily).
* If a periodic sweep find the counter at 2, it is decremented to 1.
* If the sweep find the counter at 1, the on-disk bit is cleared and the
* and sets the on-disk bit (lazily).
* If a periodic sweep find the counter at 2, it is decremented to 1.
* If the sweep find the counter at 1, the on-disk bit is cleared and the