Re: Inconsistent DB data in Streaming Replication

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>
Cc: Ants Aasma <ants(at)cybertec(dot)at>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, Sameer Thakur <samthakur74(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila(at)huawei(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, sthomas(at)optionshouse(dot)com, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Samrat Revagade <revagade(dot)samrat(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inconsistent DB data in Streaming Replication
Date: 2013-04-12 11:47:23
Message-ID: 20130412114723.GE5766@alap2.anarazel.de
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On 2013-04-12 16:58:44 +0530, Pavan Deolasee wrote:
> On Fri, Apr 12, 2013 at 4:29 PM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:
>
> >
> >
> > I don't think that holds true at all. If you look at pg_stat_bgwriter in
> > any remotely bugs cluster with a hot data set over shared_buffers you'll
> > notice that a large percentage of writes will have been done by backends
> > themselves.
> >
>
> Even if what you are saying it true, which I am sure is, the pages that the
> backend is evicting mustn't be recently used by the LRU algorithm which
> means that the WAL pertaining to the last change to the page in most
> likelihood is already replicated, unless the replication is really lagging
> much behind. Of course, if the standby is not able to keep pace with the
> master in a realistic manner then we have a problem with the approach.

It frequently takes time in the sub to few second range for usagecounts
in zero.

Greetings,

Andres Freund

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

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