Re: pg_upgrade - add config directory setting

Mr. Aaron W. Swenson wrote:
-- Start of PGP signed section.
> On Thu, Sep 29, 2011 at 10:44:29AM -0300, Alvaro Herrera wrote:
> > 
> > Excerpts from Bruce Momjian's message of jue sep 29 09:56:09 -0300 2011:
> > > 
> > > Thinking some more, I don't need to know the data directory while the
> > > server is down --- I already am starting it. pg_upgrade starts both old
> > > and new servers during its check phase, and it could look up the
> > > data_directory GUC variable and use that value when accessing files. 
> > > That would work for old and new servers.  However, I assume that is
> > > something we would not backpatch to 9.1.  
> > 
> > Why not?  We've supported separate data/config dirs for a long time now,
> > so it seems to me that pg_upgrade not coping with them is a bug.  If
> > pg_upgrade starts postmaster, it seems simple to grab the data_directory
> > setting, is it not?
> 
> I was going to say. I'd view this as bringing the behavior of pg_upgrade
> to a consistent state with postgres. I vote for it being backpatched to
> 9.0 even. For whatever my vote is worth.

Well, I would call it more of a limitation of pg_upgrade, rather than a
bug --- perhaps documenting the limitation in the back branches is
sufficient.  I think the only big argument for backpatching the feature
is that 9.1 is the first release that packagers are going to use
pg_upgrade, and fixing it now makes sense because it avoids packagers
from implementing a workaround on every platform that will go away with
9.2.

So, there are four options:

1  document the limitation and require users to use symlinks
2  add a --old/new-configdir parameter to pg_upgrade
3  have pg_upgrade find the real data dir by starting the server
4  add a flag to some tool to return the real data dir, and backpatch  that

#4 has the problem of backpatching.  I looked at #3 and the first thing
pg_upgrade does is to read PG_VERSION, and the second thing is to call
pg_controldata, both of which need the real data dir, so it is going to
require some major code ordering adjustments to do #3.

One interesting trick would be to start the old and new servers to pull
the data dir at the start only if we don't see PG_VERSION in the
specified data directory --- that would limit the overhead of starting
the servers, and limit the risk for backpatching.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +