Re: Postgres, fsync, and OSs (specifically linux)

On Fri, Apr 27, 2018 at 04:10:43PM -0700, Andres Freund wrote:
> Hi,
>
> On 2018-04-27 19:04:47 -0400, Bruce Momjian wrote:
> > On Fri, Apr 27, 2018 at 03:28:42PM -0700, Andres Freund wrote:
> > > - We need more aggressive error checking on close(), for ENOSPC and
> > > EIO. In both cases afaics we'll have to trigger a crash recovery
> > > cycle. It's entirely possible to end up in a loop on NFS etc, but I
> > > don't think there's a way around that.
> >
> > If the no-space or write failures are persistent, as you mentioned
> > above, what is the point of going into crash recovery --- why not just
> > shut down?
>
> Well, I mentioned that as an alternative in my email. But for one we
> don't really have cases where we do that right now, for another we can't
> really differentiate between a transient and non-transient state. It's
> entirely possible that the admin on the system that ran out of space
> fixes things, clearing up the problem.

True, but if we get a no-space error, odds are it will not be fixed at
the time we are failing. Wouldn't the administrator check that the
server is still running after they free the space?

> > Also, since we can't guarantee that we can write any persistent state
> > to storage, we have no way of preventing infinite crash recovery
> > loops, which, based on inconsistent writes, might make things worse.
>
> How would it make things worse?

Uh, I can imagine some writes working and some not, and getting things
more inconsistent. I would say at least that we don't know.

> > An additional features we have talked about is running some kind of
> > notification shell script to inform administrators, similar to
> > archive_command. We need this too when sync replication fails.
>
> To me that seems like a feature independent of this thread.

Well, if we are introducing new panic-and-not-restart behavior, we might
need this new feature.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +