Re: Some thoughts on NFS

Hi,

On 2019-02-19 13:21:21 -0500, Robert Haas wrote:
> On Tue, Feb 19, 2019 at 1:17 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2019-02-19 16:59:35 +0100, Magnus Hagander wrote:
> > > There might be a use-case for the split that you mention, absolutely, but
> > > it's not going to solve the people-who-want-NFS situation. You'd solve more
> > > of that by having the middle layer speak "raw device" underneath and be
> > > able to sit on top of things like iSCSI (yes, really).
> >
> > There's decent iSCSI implementations in several kernels, without the NFS
> > problems. I'm not sure what we'd gain by reimplementing those?
>
> Is that a new thing? I ran across PostgreSQL-over-iSCSI a number of
> years ago and the evidence strongly suggested that it did not reliably
> report disk errors back to PostgreSQL, leading to corruption.

How many years ago are we talking? I think it's been mostly robust in
the last 6-10 years, maybe? But note that the postgres + linux fsync
issues would have plagued that use case just as well as it did local
storage, at a likely higher incidence of failures (i.e. us forgetting to
retry fsyncs in checkpoints, and linux throwing away dirty data after
fsync failure would both have caused problems that aren't dependent on
iSCSI). And I think it's not that likely that we'd not screw up a
number of times implementing iSCSI ourselves - not to speak of the fact
that that seems like an odd place to focus development on, given that
it'd basically require all the infrastructure also needed for local DIO,
which'd likely gain us much more.

Greetings,

Andres Freund