I/O wait refers to the time that the operating system waits for an I/O operation to complete. It has a strongly positive correlation with page faults. If we see an I/O wait increasing over time, it's a strong indication that page faults will follow, as well. We should aim to keep the I/O wait at less than 60% to 70% for a healthy operational cluster. Aiming for a threshold like this will buy us some time to upgrade in the case of a suddenly increased load.