PostgreSQL Benchmarks with pg_test_fsync on a Samsung 970 Evo 1TB PCIe M.2 2280 SSD

pg_test_fsync

$ uname -a
Linux jay 4.16.12-1-ARCH #1 SMP PREEMPT Fri May 25 23:30:31 UTC 2018 x86_64 GNU/Linux
$ cat /etc/issue 
Arch Linux \r (\l)
$ pg_config --version
PostgreSQL 10.4
$ cd /var/lib/postgres/data/

$ 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                       958.857 ops/sec    1043 usecs/op
        fdatasync                           947.106 ops/sec    1056 usecs/op
        fsync                               318.890 ops/sec    3136 usecs/op
        fsync_writethrough                              n/a
        open_sync                           320.771 ops/sec    3117 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                       478.650 ops/sec    2089 usecs/op
        fdatasync                           844.952 ops/sec    1183 usecs/op
        fsync                               310.101 ops/sec    3225 usecs/op
        fsync_writethrough                              n/a
        open_sync                           159.392 ops/sec    6274 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           308.730 ops/sec    3239 usecs/op
         2 *  8kB open_sync writes          157.142 ops/sec    6364 usecs/op
         4 *  4kB open_sync writes           79.251 ops/sec   12618 usecs/op
         8 *  2kB open_sync writes           39.022 ops/sec   25627 usecs/op
        16 *  1kB open_sync writes           19.467 ops/sec   51368 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                 373.968 ops/sec    2674 usecs/op
        write, close, fsync                 308.793 ops/sec    3238 usecs/op

Non-sync'ed 8kB writes:
        write                            436554.851 ops/sec       2 usecs/op