Re: [ADMIN] Question about multiple postmasters

> On Thu, 1 Oct 1998, Bruce Momjian wrote:
>
> > > > > Has anyone tried running multiple postmasters w/ the same data
> > > > > directory.
> > > > > Any issues to watch out for?  Problems with locking?
> > > > > All of the systems would be running on Red Hat 5.1 x86.
> > > >
> > > > Hard to imagine why you would want to do that.  pg_log would be shared
> > > > by all backends, even if they connect to different databases, so this
> > > > is
> > > > very bad.
> > > Where is the pg_log located?  The only thing they would share would be
> > > data directories.  I plan on running each backing on different machines
> > > sharing a data directory.  It's more for load distribution than anything
> > > else.
> >
> > It will not work.  pg_log is in the toplevel data directory, and some of
> > the stuff is stored in shared memory, which is not shared among
> > postmasters.
>
>
> I believe what he is asking is if he can use an NFS mount or some other
> network based file system to house the data directory used by a few
> machines thereby spreading the queries across them.  Now I don't know much
> about the internals but I can suggest that he be very careful... NFS is
> basically stateless and it would be *REALLY* easy to screw up a data file.
>
> Regardless his original question still stands - can it be done?
>
> BTW replication would probably be the real solution he wants.  I could see
> trying to NFS mount a common disk area as a poor man's replication, but
> then the reliability of the entire server array becomes focused on a
> single server (the NFS master) and speed becomes based on the slowest
> link- transfering huge files over NFS.  Sort of defeats the purpose of an
> sharing the load across a set of servers doesn't it?

Can't be done.  Period.

There is a shared memory are that holds the most recent table contents.
No way of one backend under one postmaster knowing about changes another
backend under another postmaster is making.

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