Re: [COMMITTERS] pgsql-server/src backend/tcop/postgres.c

Thomas, have you commented on the objections to this patch?  If so, I
didn't see it.

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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Thomas Lockhart wrote:
> >> Implement WAL log location control using "-X" or PGXLOG.
> 
> > Woh, I didn't think we had agreement on this.  This populates the 'X'
> > all over the system (postgres, postmaster, initdb, pg_ctl),  while the
> > simple solution would be to add the flag only to initdb and use a
> > symlink from /data.  I also think it is less error-prone because you
> > can't accidentally point to the wrong XLOG directory.  In fact, you
> > almost have to use an environment variable unles you plan to specify -X
> > for all the commands.   In my mind, PGDATA should take care of the whole
> > thing for pointing to your data.
> 
> > If you want to do it this way, I request a vote.
> 
> I have to vote a strong NO on this.  What the patch essentially does is
> to decouple specification of the data directory ($PGDATA or -D) from the
> xlog directory ($PGXLOG or -X).  This might seem nice and clean and
> symmetrical, but in fact it has no conceivable use except for shooting
> yourself in the foot in a particularly nasty manner.  The xlog *cannot*
> be decoupled from the data directory --- there are pointers in
> pg_control and in every single data page that depend on the state of
> xlog.  Trying to make them look independent is just a recipe for
> breaking your database by starting the postmaster with the wrong
> combination of PGDATA and PGXLOG.  And I'm not at all sure it'll be
> possible to recover after you do that: if the postmaster notices the
> discrepancy, it is likely to try to fix it by rolling forward from the
> last checkpoint it can find in the mismatching xlog.  Oops :-(
> 
> I think the existing mechanism of using a symlink in the data directory
> when you want to move xlog is far safer and more reliable.  I do not see
> what functionality is added by this patch that can possibly justify the
> hazards it creates.
> 
> 			regards, tom lane
> 

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