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Re: Streaming replication, and walsender during recovery

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Streaming replication, and walsender during recovery
Date: 2010-01-28 18:29:23
Message-ID: 1264703363.24669.11354.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, 2010-01-28 at 13:05 -0500, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > On Thu, 2010-01-28 at 11:41 -0500, Tom Lane wrote:
> >> FWIW, I don't agree with that prioritization in the least.  Cascading
> >> is something we could leave till 9.1, or even later, and 
> 
> > Not what you said just a few days ago.
> 
> Me?  I don't recall having said a word about cascading before.

Top of this thread.

> > I'm a little worried the feature set of streaming rep isn't any better
> > than what we have already.
> 
> Nonsense.  Getting rid of the WAL-segment-based shipping delays is a
> quantum improvement --- it means we actually have something approaching
> real-time replication, which was really impractical before.  Whether you
> can feed slaves indirectly is just a minor administration detail.  Yeah,
> I know in some situations it could be helpful for performance, but
> it's not even in the same ballpark of must-have-ness.

FWIW, streaming has been possible and actively used since 8.2. 

> (Anyway, the argument that it's important for performance is pure
> speculation AFAIK, untainted by any actual measurements.  Given the lack
> of optimization of WAL replay, it seems entirely possible that the last
> thing you want to burden a slave with is sourcing data to more slaves.)

Separate processes, separate CPUs, no problem. If WAL replay used more
CPUs you might be right, but it doesn't yet, so same argument opposite
conclusion.

-- 
 Simon Riggs           www.2ndQuadrant.com


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