RE: [HACKERS][PATCH] Applying PMDK to WAL operations for persistentmemory

From: Robert Haas [mailto:robertmhaas@gmail.com]
> I think open_datasync will be worse on systems where fsync() is expensive
> -- it forces the data out to disk immediately, even if the data doesn't
> need to be flushed immediately.  That's bad, because we wait immediately
> when we could have deferred the wait until later and maybe gotten the WAL
> writer to do the work in the background.  But it might be better on systems
> where fsync() is basically free, because there you might as well just get
> it out of the way immediately and not leave something left to be done later.
> 
> This is just a guess, of course.  You didn't mention what the underlying
> storage for your test was?

Uh, your guess was correct.  My file system was ext3, where fsync() writes all dirty buffers in page cache.

As you said, open_datasync was 20% faster than fdatasync on RHEL7.2, on a LVM volume with ext4 (mounted with options
noatime,nobarrier) on a PCIe flash memory.
 

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                     50829.597 ops/sec      20 usecs/op
        fdatasync                         42094.381 ops/sec      24 usecs/op
        fsync                                          42209.972 ops/sec      24 usecs/op
        fsync_writethrough                            n/a
        open_sync                         48669.605 ops/sec      21 usecs/op

Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync is Linux's default)
        open_datasync                     26366.373 ops/sec      38 usecs/op
        fdatasync                         33922.725 ops/sec      29 usecs/op
        fsync                             32990.209 ops/sec      30 usecs/op
        fsync_writethrough                            n/a
        open_sync                         24326.249 ops/sec      41 usecs/op

What do you think about changing the default value of wal_sync_method on Linux in PG 11?  I can understand the concern
thatusers might hit performance degredation if they are using PostgreSQL on older systems.  But it's also mottainai
thatmany users don't notice the benefits of wal_sync_method = open_datasync on new systems.
 

Regards
Takayuki Tsunakawa