Amit Kapila wrote: > On Fri, Jun 1, 2018 at 3:13 PM, Laurenz Albe <laurenz.albe@cybertec.at> wrote: > > I recently read our documentation about reliability on Windows: > > > > > On Windows, if wal_sync_method is open_datasync (the default), write caching can > > > be disabled by unchecking > > > My Computer\Open\disk drive\Properties\Hardware\Properties\Policies\Enable write caching > > > on the disk. Alternatively, set wal_sync_method to fsync or fsync_writethrough, > > > which prevent write caching. > > > > It seems dangerous to me to initialize "wal_sync_method" to a method that is unsafe > > by default. Admittedly I am not a Windows man, but the fact that this has eluded me > > up to now leads me to believe that other people running PostgreSQL on Windows might > > also have missed that important piece of advice and are consequently running with > > an unsafe setup. > > > > Wouldn't it be smarter to set a different default value on Windows, like we do on > > Linux (for other reasons)? > > > > One thing to note is that it seems that in code we use FILE_FLAG_WRITE_THROUGH for > open_datasync which according to MSDN [1] will bypass any intermediate cache . > See pgwin32_open. Have you experimented to set any other option as we have a comment > in code which say Win32 only has O_DSYNC? > > > [1] - https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx
After studying the code I feel somewhat safer; it looks like the code is ok. I have no Windows at hand, so I cannot test right now.
What happened is that I ran "pg_test_fsync" at a customer site on Windows, and it returned ridiculously high rates got open_datasync.
So I think that the following should be fixed:
- Change pg_test_fsync to actually use FILE_FLAG_WRITE_THROUGH.
It sounds sensible to me to make a Windows specific change in pg_test_fsync for open_datasync method. That will make pg_test_fsync behave similar to server.