Re: Fails to work on live images due to fsync() on pg_commit_ts before doing any write there

On 2017-11-07 16:06:02 +0100, Raphael Hertzog wrote:
> On Tue, 07 Nov 2017, Stephen Frost wrote:
> > Raphael,
> >
> > * Raphael Hertzog (hertzog(at)debian(dot)org) wrote:
> > > Still I believe that this issue should be fixed in both sides. It's not
> > > smart from PostgreSQL to call fsync() when it has not made any change.
> >
> > Why?
>
> Because it's useless. Why call fsync() when you know that it doesn't do
> anything?

It's not generally useless, no. You don't know if the relevant directory
hasn't been modified since postgres was started last / crashed
last. That matters for the durability and atomicity of renames
etc. There's simply no way to track that in postgres, given it's
arbitrary external effects.

Greetings,

Andres Freund