Subject: Re: rmgr hooks and contrib/rmgr_hook
From: Simon Riggs <simon@2ndQuadrant.com>
To: ITAGAKI Takahiro <itagaki.takahiro@oss.ntt.co.jp>
Cc: pgsql-hackers@postgresql.org
In-Reply-To: <20080902174613.76B2.52131E4D@oss.ntt.co.jp>
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	<1220345064.4371.275.camel@ebony.2ndQuadrant>
	<20080902174613.76B2.52131E4D@oss.ntt.co.jp>
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Date: Tue, 02 Sep 2008 10:49:40 +0100
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On Tue, 2008-09-02 at 18:30 +0900, ITAGAKI Takahiro wrote:
> Simon Riggs <simon@2ndQuadrant.com> wrote:
> 
> > > Why do we need to set rmgr_hook in _PG_init(), and add or mofify rmgrs
> > > in our hook functions?
> > 
> > If we modify RmgrTable in _PG_init() then we would have to have that
> > structure available in all backends, which was a stated objective to
> > avoid. We would still need a fast access data structure for the
> > XLogInsert() check, so the RmgrTable would just be wasted space in all
> > normal backends. In the patch, plugin is only called when we call
> > RmgrInitialize(), so the memory is malloc'd only when required.
> 
> Um? AFAICS RmgrTable is not accessed in XLogInsert unless we use WAL_DEBUG.

Exactly why I want to malloc it.

> I see that RmgrTable should be malloc'd when required,
> but there is another issue; when to load rmgr libraries.

> Rmgr objects are needed only in startup process during recovery.
> If we want to reduce resource consumption by rmgrs, I think it is
> better not to load rmgr libraries through shared_preload_libraries.
> We don't have to load rmgr libs if recovery is not needed or after recovery.
> 
> How about adding a new variable "recovery_preload_libaries" like as
> shared_preload_libraries? Rmgr libs in it are loaded only in startup
> process and only if recovery is needed.

Good point. If others agree, I will re-implement this way.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support