Re: PostgreSQL's handling of fsync() errors is unsafe and risks data loss at least on XFS

On Wed, Apr 4, 2018 at 01:54:50PM +1200, Thomas Munro wrote:
> On Wed, Apr 4, 2018 at 12:56 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > There has been a lot of focus in this thread on the workflow:
> >
> > write() -> blocks remain in kernel memory -> fsync() -> panic?
> >
> > But what happens in this workflow:
> >
> > write() -> kernel syncs blocks to storage -> fsync()
> >
> > Is fsync() going to see a "kernel syncs blocks to storage" failure?
> >
> > There was already discussion that if the fsync() causes the "syncs
> > blocks to storage", fsync() will only report the failure once, but will
> > it see any failure in the second workflow? There is indication that a
> > failed write to storage reports back an error once and clears the dirty
> > flag, but do we know it keeps things around long enough to report an
> > error to a future fsync()?
> >
> > You would think it does, but I have to ask since our fsync() assumptions
> > have been wrong for so long.
>
> I believe there were some problems of that nature (with various
> twists, based on other concurrent activity and possibly different
> fds), and those problems were fixed by the errseq_t system developed
> by Jeff Layton in Linux 4.13. Call that "bug #1".

So all our non-cutting-edge Linux systems are vulnerable and there is no
workaround Postgres can implement? Wow.

> The second issues is that the pages are marked clean after the error
> is reported, so further attempts to fsync() the data (in our case for
> a new attempt to checkpoint) will be futile but appear successful.
> Call that "bug #2", with the proviso that some people apparently think
> it's reasonable behaviour and not a bug. At least there is a
> plausible workaround for that: namely the nuclear option proposed by
> Craig.

Yes, that one I understood.

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

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