Re: Add pg_file_sync() to adminpack

On Thu, Jan 9, 2020 at 6:16 PM Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>
> Greetings,
>
> * Julien Rouhaud (rjuju123(at)gmail(dot)com) wrote:
> > On Thu, Jan 9, 2020 at 7:43 AM Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> > > On Wed, Dec 25, 2019 at 11:11 PM Julien Rouhaud <rjuju123(at)gmail(dot)com> wrote:
> > > > On Wed, Dec 25, 2019 at 2:01 PM Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> > > > > I'd like to propose to add pg_file_sync() function into contrib/adminpack.
> > > > > This function fsyncs the specified file or directory named by its argument.
> > > > > IMO this is useful, for example, when you want to fsync the file that
> > > > > pg_file_write() writes out or that COPY TO exports the data into,
> > > > > for durability. Thought?
> > > >
> > > > +1, that seems like a useful wrapper. Looking at existing functions,
> > > > I see that there's a pg_file_rename() in adminpack, but it doesn't use
> > > > durable_rename nor does it try to perform any fsync. Same for
> > > > pg_file_unlink vs. durable_unlink. It's probably worth fixing that at
> > > > the same time?
> > >
> > > I don't think that's a bug. I'm not sure if every users of those functions
> > > need durable rename and unlink at the expense of performance.
> > > So IMO it's better to add new argument like "durable" to those functions
> > > and durable_rename or _unlink is used only if it's true.
> >
> > It's probably a POLA violation. I'm pretty sure that most people
> > using those functions would expect that a successful call to
> > pg_file_unlink() mean that the file cannot raise from the dead even
> > with certain unlikely circumstances, at least I'd expect so. If
> > performance is a problem here, I'd rather have a new wrapper with a
> > sync flag that defaults to true so it's possible to disable it if
> > needed instead of calling a different function. That being said, I
> > agree with Arthur, it should be handled in a different patch.
>
> Why would you expect that when it isn't the case for the filesystem
> itself..?

Just a usual habit with durable property.

> I agree with Fujii on this- you should have to explicitly ask
> for us to do more than the equivilant filesystem-level operation. We
> shouldn't be forcing that on you.

I just checked other somehow related cases and saw that for instance
COPY TO doesn't flush data either, so it's indeed the expected
behavior.