Re: pg_receivexlog sometimes fails on first start

On Fri, Feb 6, 2015 at 6:17 AM, Heikki Linnakangas
<hlinnakangas(at)vmware(dot)com> wrote:
> When starting pg_receivexlog for the first time, i.e. with an empty
> directory, it sometimes fails like this:
>
> pg_receivexlog: unexpected termination of replication stream: ERROR:
> requested starting point 56/70000000 is ahead of the WAL flush position of
> this server 56/6FFC4000
>
> This is easier to reproduce if you have a steady stream of updates, with no
> commits, running concurrently. A bulk load, for example.
>
> The problem is that pg_receivexlog issues the IDENTIFY_SYSTEM command, which
> returns the current WAL insert position. It then requests to start streaming
> from that position. Or to be precise, from the beginning of the WAL segment
> containing that position. That's wrong; the WAL might not be flushed up to
> the insert position yet.
>
> I think returning the insert position in IDENTIFY_SYSTEM is a bad idea. We
> even mention in the documentation that that field is "useful to get a known
> location in the transaction log where streaming can start", but the insert
> position is wrong for that purpose.

Good catch.

> Any objections to changing IDENTIFY_SYSTEM to return the flush position,
> instead of insert position? We haven't heard any complaints from the field
> about this, but IMHO that should also be back-patched. It's a design bug,
> and the fix is simple and unlikely to cause any harm.

+1 with your idea.

Regards,

--
Fujii Masao