RE: PgSQL 12 on WinSrv ~3x faster than on Linux

> The first thing that comes to mind for me is fsync working correctly (i.e.
> actually waiting for the disk write) in Linux but not in Windows.
> On a weird VM stack like you've got, it's not hard for that sort of thing to go
> wrong.  Needless to say, if that's the issue then the apparent performance
> win is coming at the cost of crash safety.
>
> pg_test_fsync might help detect such a problem.
>
>             regards, tom lane
>

fsync performance on win is much better (results are below). Also network performance for VMs on same HV for win-win is
40%better than for win-linux (5,97Gbps vs 3,6Gbps). 
Regarding weird VM stack - we're running both win and linux VMs and Hyper-V works reasonable well except... this issue
)

Win:
C:\Program Files\PostgreSQL\12.6-6.1C\bin>pg_test_fsync.exe
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                      7423,645 ops/sec     135 usecs/op
        fdatasync                                       n/a
        fsync                              1910,611 ops/sec     523 usecs/op
        fsync_writethrough                 1987,900 ops/sec     503 usecs/op
        open_sync                                       n/a

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                      3827,254 ops/sec     261 usecs/op
        fdatasync                                       n/a
        fsync                              1920,720 ops/sec     521 usecs/op
        fsync_writethrough                 1863,852 ops/sec     537 usecs/op
        open_sync                                       n/a

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                       n/a
         2 *  8kB open_sync writes                      n/a
         4 *  4kB open_sync writes                      n/a
         8 *  2kB open_sync writes                      n/a
        16 *  1kB open_sync writes                      n/a

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                 144,065 ops/sec    6941 usecs/op
        write, close, fsync                 148,751 ops/sec    6723 usecs/op

Non-sync'ed 8kB writes:
        write                               165,484 ops/sec    6043 usecs/op

Linux:
[root@pgsql12 ~]# /usr/pgsql-12/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                      2947.296 ops/sec     339 usecs/op
        fdatasync                          2824.271 ops/sec     354 usecs/op
        fsync                              1885.924 ops/sec     530 usecs/op
        fsync_writethrough                              n/a
        open_sync                          1816.312 ops/sec     551 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                      1458.849 ops/sec     685 usecs/op
        fdatasync                          2712.756 ops/sec     369 usecs/op
        fsync                              1769.353 ops/sec     565 usecs/op
        fsync_writethrough                              n/a
        open_sync                           902.626 ops/sec    1108 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          1798.811 ops/sec     556 usecs/op
         2 *  8kB open_sync writes          887.727 ops/sec    1126 usecs/op
         4 *  4kB open_sync writes          494.843 ops/sec    2021 usecs/op
         8 *  2kB open_sync writes          233.659 ops/sec    4280 usecs/op
        16 *  1kB open_sync writes          117.417 ops/sec    8517 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                1673.781 ops/sec     597 usecs/op
        write, close, fsync                1727.787 ops/sec     579 usecs/op

Non-sync'ed 8kB writes:
        write                            200638.271 ops/sec       5 usecs/op

--
Taras