$ uname -a
Linux owl 5.5.8-arch1-1 #1 SMP PREEMPT Fri, 06 Mar 2020 00:57:33 +0000 x86_64 GNU/Linux
$ cat /etc/os-release 
NAME="Arch Linux"
PRETTY_NAME="Arch Linux"
ID=arch
BUILD_ID=rolling
ANSI_COLOR="0;36"
HOME_URL="https://www.archlinux.org/"
DOCUMENTATION_URL="https://wiki.archlinux.org/"
SUPPORT_URL="https://bbs.archlinux.org/"
BUG_REPORT_URL="https://bugs.archlinux.org/"
$ pg_config --version
PostgreSQL 12.2

$ pg_test_fsync
5 seconds per test
O_DIRECT supported on this platform for open_datasync and open_sync.

Compare file sync methods using one 8kB write:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                       580.333 ops/sec    1723 usecs/op
        fdatasync                          1435.984 ops/sec     696 usecs/op
        fsync                               682.364 ops/sec    1465 usecs/op
        fsync_writethrough                              n/a
        open_sync                           688.987 ops/sec    1451 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                       289.439 ops/sec    3455 usecs/op
        fdatasync                          1326.101 ops/sec     754 usecs/op
        fsync                               659.379 ops/sec    1517 usecs/op
        fsync_writethrough                              n/a
        open_sync                           344.222 ops/sec    2905 usecs/op

Compare open_sync with different write sizes:
(This is designed to compare the cost of writing 16kB in different write
open_sync sizes.)
         1 * 16kB open_sync write           659.935 ops/sec    1515 usecs/op
         2 *  8kB open_sync writes          344.816 ops/sec    2900 usecs/op
         4 *  4kB open_sync writes          177.187 ops/sec    5644 usecs/op
         8 *  2kB open_sync writes           82.043 ops/sec   12189 usecs/op
        16 *  1kB open_sync writes           41.345 ops/sec   24187 usecs/op

Test if fsync on non-write file descriptor is honored:
(If the times are similar, fsync() can sync data written on a different
descriptor.)
        write, fsync, close                 679.352 ops/sec    1472 usecs/op
        write, close, fsync                 525.375 ops/sec    1903 usecs/op

Non-sync'ed 8kB writes:
        write                            433558.074 ops/sec       2 usecs/op