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. Currently it uses O_DSYNC, which is defined in port/win32_port.h as /* * Supplement to <fcntl.h>. * This is the same value as _O_NOINHERIT in the MS header file. This is * to ensure that we don't collide with a future definition. It means * we cannot use _O_NOINHERIT ourselves. */ #define O_DSYNC 0x0080 - Change the documentation so that it does not claim that open_datasync is unsafe unless you change the device settings. If there is a consensus that this is fine, I can do the legwork. Yours, Laurenz Albe
