Need to know more about pg_test_fsync utility

Hi Team,

I would like to understand the internals of pg_test_fsync utility as I know
pg_test_fsync is intended to give us a reasonable idea of what is the
fastest wal_sync_method for our system but as per the documentation it will
also provide the diagnostic information in the event of an I/O problem. So,
I have below questions:

1) How to interpret the output of pg_test_fsync?
2) What is the meaning of ops/sec & usecs/op?
3) How does this utility work internally?
4) What is the IO pattern of this utility? serial/sequence IO or Multiple
thread with Parallel IO?
5) Can we change the testing like FIO with multiple threads and parallel IO?
6) How a commit happened in the background while executing this utility?
7) How can we use this tool to measure the I/O issue?
8) In which area or section in the output do we need to focus while
troubleshooting I/O issues?
9) What is the meaning of “Non-sync’ed 8kB writes?

I also would like to know is there any best Practice from PostgreSQL which
mentions what is the disk latency required for the WAL & DATA disk?

Thank You :)