The reason this happens is because due to memory pressure, the IO cache is emptied and thus every IO request (even if reading just one byte) is passed through t

The reason this happens is because due to memory pressure, the IO cache is emptied and thus every IO request (even if reading just one byte) is passed through to the actual storage device. This quickly fills up the device’s queue and IO performance can no longer keep up with the system’s demands even for normally-benign stuff such as writing logs.