Re: rmgr hooks and contrib/rmgr_hook

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: rmgr hooks and contrib/rmgr_hook
Date: 2008-09-02 09:49:40
Message-ID: 1220348980.4371.316.camel@ebony.2ndQuadrant
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On Tue, 2008-09-02 at 18:30 +0900, ITAGAKI Takahiro wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)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
PostgreSQL Training, Services and Support

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