Re: Standalone synchronous master

On 2014-01-10 17:02:08 -0500, Stephen Frost wrote:
> * Andres Freund (andres@2ndquadrant.com) wrote:
> > On 2014-01-10 10:59:23 -0800, Joshua D. Drake wrote:
> > > If a synchronous slave goes down, the master continues to operate. That is
> > > all. I don't care if it is configurable (I would be fine with that). I don't
> > > care if it is not automatic (e.g; slave goes down and we have to tell the
> > > master to continue).
> > 
> > Would you please explain, as precise as possible, what the advantages of
> > using a synchronous standby would be in such a scenario?
> 
> In a degraded/failure state, things continue to *work*.  In a
> non-degraded/failure state, you're able to handle a system failure and
> know that you didn't lose any transactions.

Why do you know that you didn't loose any transactions? Trivial network
hiccups, a restart of a standby, IO overload on the standby all can
cause a very short interruptions in the walsender connection - leading
to degradation.

> As pointed out by someone
> previously, that's how RAID-1 works (which I imagine quite a few of us
> use).

I don't think that argument makes much sense. Raid-1 isn't safe
as-is. It's only safe if you use some sort of journaling or similar
ontop. If you issued a write during a crash you normally will just get
either the version from before or the version after the last write back,
depending on the state on the individual disks and which disk is treated
as authoritative by the raid software.

And even if you disregard that, there's not much outside influence that
can lead to loosing connection to a disk drive inside a raid outside an
actually broken drive. Any network connection is normally kept *outside*
the leven at which you build raids.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services