Re: Re: BUG #13685: Archiving while idle every archive_timeout with wal_level hot_standby

From: Andres Freund <andres(at)anarazel(dot)de>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Re: BUG #13685: Archiving while idle every archive_timeout with wal_level hot_standby
Date: 2016-02-11 08:20:17
Message-ID: 20160211082017.peovlrntnqeisji6@alap3.anarazel.de
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On 2016-02-11 09:25:30 +0530, Amit Kapila wrote:
> On Wed, Feb 10, 2016 at 1:42 PM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
> wrote:
> >
> > On Wed, Feb 10, 2016 at 5:00 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
> > > Okay, but isn't it better that we remove the snapshot taken
> > > at checkpoint time in the main branch or till where this code is
> > > getting back-patched. Do you see any need of same after
> > > having the logging of snapshot in bgwriter?
> >
> > But this one is necessary as well to allow hot standby faster to
> > initialize, no? Particularly in the case where a bgwriter snapshot
> > would have been taken just before the checkpoint, there may be up to
> > 15s until the next one.
> >
>
> It could be helpful if checkpoint is done at shorter intervals, otherwise
> we anyway log it at 15s interval and if need faster initialisation
> of hot-standby, then it is better to reduce the log snapshot interval
> in bgwriter.

No. By emitting the first snapshot directly after the determination of
the redo pointer, we can get into STANDBY_SNAPSHOT_READY more
quickly. Especially if some of the snapshots are overflowed. During
startup 15s can be a *long* time; but on the other hand there's not much
benefit at logging snapshots more frequently when the system is up.

I don't think we should tinker with the frequency/logging points, while
fixing the issue here.

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