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Re: Hot standby, recovery infra

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hot standby, recovery infra
Date: 2009-01-30 10:47:55
Message-ID: 1233312475.8859.7.camel@ebony.2ndQuadrant (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 2009-01-29 at 19:20 +0200, Heikki Linnakangas wrote:
> Heikki Linnakangas wrote:
> > It looks like if you issue a fast shutdown during recovery, postmaster 
> > doesn't kill bgwriter.
> Hmm, seems like we haven't thought through how shutdown during 
> consistent recovery is supposed to behave in general. Right now, smart 
> shutdown doesn't do anything during consistent recovery, because the 
> startup process will just keep going. And fast shutdown will simply 
> ExitPostmaster(1), which is clearly not right.

That whole area was something I was leaving until last, since immediate
shutdown doesn't work either, even in HEAD. (Fujii-san and I discussed
this before Christmas, briefly).

> I'm thinking that in both smart and fast shutdown, the startup process 
> should exit in a controlled way as soon as it's finished with the 
> current WAL record, and set minSafeStartPoint to the current point in 
> the replay.

That makes sense, though isn't required.

> I wonder if bgwriter should perform a restartpoint before exiting? 
> You'll have to start with recovery on the next startup anyway, but at 
> least we could minimize the amount of WAL that needs to be replayed.

That seems like extra work for no additional benefit.

I think we're beginning to blur the lines between review and you just
adding some additional stuff in this area. There's nothing to stop you
doing further changes after this has been committed. We can also commit
what we have with some caveats also, i.e. commit in pieces.

 Simon Riggs 
 PostgreSQL Training, Services and Support

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