Observation

In case of the page cache I/O, the quiesce times (upper red graph) significantly rise as the actual fio throughput increases.

In the direct I/O case (lower red graph) the influence of the filesystem I/O workload on the quiesce time is lower for low fio throughputs, but rises sharply for the maximum throughput of 708 MiB/s.

This is different for the amount of pages transferred during the relocation process: While that number roughly scales with the actual fio throughput in the page cache I/O case (upper blue graph), in the direct I/O case there is no visible influence of the actual fio throughput on the amount of pages transferred (lower blue graph).