Postgres / PostgreSQL Benchmarks with pg_test_fsync on a 4-drive Raid 5 of WDC WD2002FYPS-0 2TB SATA HDDs (Intel i7-4770K and Intel Z87 Chipset)

pg_test_fsync

$ uname -a
Linux owl 5.4.8-arch1-1 #1 SMP PREEMPT Sat, 04 Jan 2020 23:46:18 +0000 x86_64 GNU/Linux
$ cat /etc/issue 
Arch Linux \r (\l)
$ pg_config --version
PostgreSQL 12.1
$ cd ~

$ 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                        89.503 ops/sec   11173 usecs/op
        fdatasync                            86.933 ops/sec   11503 usecs/op
        fsync                                30.719 ops/sec   32553 usecs/op
        fsync_writethrough                              n/a
        open_sync                            29.384 ops/sec   34032 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                        44.177 ops/sec   22636 usecs/op
        fdatasync                            88.999 ops/sec   11236 usecs/op
        fsync                                28.369 ops/sec   35249 usecs/op
        fsync_writethrough                              n/a
        open_sync                            14.251 ops/sec   70168 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            29.645 ops/sec   33732 usecs/op
         2 *  8kB open_sync writes           14.815 ops/sec   67498 usecs/op
         4 *  4kB open_sync writes            7.247 ops/sec  137983 usecs/op
         8 *  2kB open_sync writes            3.579 ops/sec  279445 usecs/op
        16 *  1kB open_sync writes            1.866 ops/sec  535834 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                  28.876 ops/sec   34631 usecs/op
        write, close, fsync                  29.316 ops/sec   34111 usecs/op

Non-sync'ed 8kB writes:
        write                            453549.074 ops/sec       2 usecs/op