gistfile1.txt

###  ex61-nvme, ubuntu 18.04 minimal, no raid. apt-get update, apt-get install postgresql
/usr/lib/postgresql/10/bin/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                       974.559 ops/sec    1026 usecs/op
        fdatasync                           957.327 ops/sec    1045 usecs/op
        fsync                               366.054 ops/sec    2732 usecs/op
        fsync_writethrough                              n/a
        open_sync                           396.387 ops/sec    2523 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                       479.559 ops/sec    2085 usecs/op
        fdatasync                           821.378 ops/sec    1217 usecs/op
        fsync                               335.078 ops/sec    2984 usecs/op
        fsync_writethrough                              n/a
        open_sync                           186.964 ops/sec    5349 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           355.013 ops/sec    2817 usecs/op
         2 *  8kB open_sync writes          197.986 ops/sec    5051 usecs/op
         4 *  4kB open_sync writes          108.016 ops/sec    9258 usecs/op
         8 *  2kB open_sync writes           47.818 ops/sec   20913 usecs/op
        16 *  1kB open_sync writes           23.681 ops/sec   42227 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                 406.618 ops/sec    2459 usecs/op
        write, close, fsync                 418.951 ops/sec    2387 usecs/op

Non-sync'ed 8kB writes:
        write                            523028.582 ops/sec       2 usecs/op
        
        
### cx11 similar setup
/usr/lib/postgresql/10/bin/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                      2862.679 ops/sec     349 usecs/op
        fdatasync                          2992.365 ops/sec     334 usecs/op
        fsync                              2556.479 ops/sec     391 usecs/op
        fsync_writethrough                              n/a
        open_sync                          2614.522 ops/sec     382 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                      1533.507 ops/sec     652 usecs/op
        fdatasync                          2721.964 ops/sec     367 usecs/op
        fsync                              2120.455 ops/sec     472 usecs/op
        fsync_writethrough                              n/a
        open_sync                          1373.114 ops/sec     728 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          2276.117 ops/sec     439 usecs/op
         2 *  8kB open_sync writes         1436.539 ops/sec     696 usecs/op
         4 *  4kB open_sync writes          710.259 ops/sec    1408 usecs/op
         8 *  2kB open_sync writes          368.992 ops/sec    2710 usecs/op
        16 *  1kB open_sync writes          182.178 ops/sec    5489 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                2599.101 ops/sec     385 usecs/op
        write, close, fsync                2670.547 ops/sec     374 usecs/op

Non-sync'ed 8kB writes:
        write                            331707.474 ops/sec       3 usecs/op