Re: Loaded footgun open_datasync on Windows

On Fri, Jun 1, 2018 at 8:18 PM, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
wrote:

> Amit Kapila wrote:
> > On Fri, Jun 1, 2018 at 3:13 PM, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)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