Am I properly forcing RocksDB to use fsync? Neither fsync() nor msync() shows in strace - rocksdb

Question

I'm using RocksDB via the C API.
I have a test program that opens a database, does 1,000 writes (gathering timing data between initiation of write and callback), does 1,000 reads, and shuts down.
This works. Average time to do a write is about 1ms.
I modified the test program to turn on write syncing via this
rocksdb_writeoptions_set_sync(wri_u, 1);
and ran it again. Average time to do a write is about 8ms.
So far, so good.
HOWEVER, I then ran strace on both versions of the program to verify that fsync() or fdatasync() or msync() is getting called.
The no-sync program shows 4 invocations of fsync(), 2 of fdatasync() and 0 of msync(). Reasonable.
...but the sync version of the program shows the same 4, 2, and 0. Odd! Surprising! Worrying!
The sync version DOES show 2 interesting deltas from the no-sync version: (i) 2 calls to nanosleep() per write, (ii) an 80% increase in the time spent in mmap().
One out-of-my-butt theory is that perhaps msync() [ or a stand-in for it ] is actually implemented in terms of nanosleep() ?
This is on a desktop Linux 16.04
uname -a
Linux mithril 4.4.0-139-generic #165-Ubuntu SMP Wed Oct 24 10:58:50 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Anyway, my question is, as per the subject line:
Am I properly forcing RocksDB to use fsync? ... because neither fsync() nor msync() shows in strace
Thanks.

Answer 1

Yes, this is the correct way to turn fsync() on.
The issue is that strace must be used with the -f flag to trace system calls in new threads ... and RocksDB was doing all syncs in other threads.