Re: Drop type "smgr"?

On Thu, Feb 28, 2019 at 02:08:49PM -0800, Andres Freund wrote:
> On 2019-03-01 09:48:33 +1300, Thomas Munro wrote:
> > On Fri, Mar 1, 2019 at 7:24 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > On 2019-02-28 13:16:02 -0500, Tom Lane wrote:
> > > > Shawn Debnath <sdn(at)amazon(dot)com> writes:
> > > > > Another thought: my colleague Anton Shyrabokau suggested potentially
> > > > > re-using forknumber to differentiate smgrs. We are using 32 bits to
> > > > > map 5 entries today.
> > > >
> > > > Yeah, that seems like it might be a workable answer.
> > >
> > > Yea, if we just split that into two 16bit entries, there'd not be much
> > > lost. Some mild mild performance regression due to more granular
> > > memory->register reads/writes, but I can't even remotely see that
> > > matter.
> >
> > Ok, that's a interesting way to include it in BufferTag without making
> > it wider. But then how about the SMGR interface? I think that value
> > would need to be added to the smgropen() interface, and all existing
> > callers would pass in (say) SGMR_RELATION (meaning "use md.c"), and
> > new code would use SMGR_UNDO, SMGR_SLRU etc. That seems OK to me.
>
> Right, seems like we should do that independent of whether we end up
> reusing the dboid or not.

Food for thought: if we are going to muck with the smgr APIs, would it
make sense to move away from SMgrRelation to something a bit more
generic like, say, SMgrHandle to better organize the internal contents
of this structure? Internally, we could move things into an union and
based on type of handle: relation, undo, slru/generic, translate the
contents correctly. As you can guess, SMgrRelationData today is very
specific to relations and holding md specific information whose memory
would be better served re-used for the other storage managers.

Thoughts?

--
Shawn Debnath
Amazon Web Services (AWS)