Re: [HACKERS] Re: pgsql: Make CheckRequiredParameterValues() depend upon correct

On Fri, Apr 23, 2010 at 4:44 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> No intention of doing that. This change allows people to see what the
>> dependency actually is once the bug has been fixed. Change needs to
>> start from here, not from where we were before.
>
> Well, actually, now that I've looked at the patch I think it's starting
> from a fundamentally wrong position anyway.  Checkpoint records are a
> completely wrong mechanism for transmitting this data to slaves, because
> a checkpoint is emitted *after* we do something, not *before* we do it.
> In particular it's ludicrous to be looking at shutdown checkpoints to
> try to determine whether the subsequent WAL will meet the slave's
> requirements.  There's no connection at all between what the GUC state
> was at shutdown and what it might be after starting again.
>
> A design that might work is
> (1) store the active value of wal_mode in pg_control (but NOT as part of
> the last-checkpoint-record image).
> (2) invent a new WAL record type that is transmitted when we change
> wal_mode.

Well, right now wal_mode would only be able to be changed at server
restart. Eventually we might relax that, but I think there are some
restrictions on how we can do it - like maybe needing to wait until
all the transactions running at the time the change was decided on
have committed, or, well, I'm not sure.

...Robert