Re: Hot Standby: Startup at shutdown checkpoint

Simon Riggs wrote:
> In StartupXlog() when we get to the point where we "Find the first
> record that logically follows the checkpoint", in the current code
> ReadRecord() loops forever, spitting out
> LOG: record with zero length at 0/C000088
> ...
>
> That prevents us from going further down StartupXLog() to the point
> where we start the InRedo loop and hence start hot standby. As long as
> we retry we cannot progress further: this is the main problem.
>
> So in the patch, I have modified the retry test in ReadRecord() so it no
> longer retries iff there is no WAL source defined. Now, when
> ReadRecord() exits, record == NULL at that point and so we do not (and
> cannot) enter the redo loop.

Oh, I see.

> So I have introduced the new mode ("snapshot mode") to enter hot standby
> anyway. That avoids us having to screw around with the loop logic for
> redo. I don't see any need to support the case of where we have no WAL
> source defined, yet we want Hot Standby but we also want to allow
> somebody to drop a WAL file into pg_xlog at some future point. That has
> no use case of value AFAICS and is too complex to add at this stage of
> the release cycle.

You don't need a new mode for that. Just do the same "are we consistent
now?" check you do in the loop once before calling ReadRecord to fetch
the record that follows the checkpoint pointer. Attached is a patch to
show what I mean. We just need to let postmaster know that recovery has
started a bit earlier, right after processing the checkpoint record, not
delaying it until we've read the first record after it.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com