* Whenever IO on the active page completes, the Ready page is activated
* and the ex-Active page is clean out and made Ready.
* Otherwise the Ready page is only activated when it becomes full, or
* Whenever IO on the active page completes, the Ready page is activated
* and the ex-Active page is clean out and made Ready.
* Otherwise the Ready page is only activated when it becomes full, or
*
* If a request arrives while both pages a full, it is queued, and b_rdev is
* overloaded to record whether it was a read or a write.
*
* If a request arrives while both pages a full, it is queued, and b_rdev is
* overloaded to record whether it was a read or a write.