On Fri, Jun 1, 2018 at 8:18 PM, Laurenz Albe <laurenz.a...@cybertec.at> wrote:
> Amit Kapila wrote: > > On Fri, Jun 1, 2018 at 3:13 PM, Laurenz Albe <laurenz.a...@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. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
