Re: data loss with pg_standby when doing a controlled failover

Scott Mead wrote:
>
>
>
> On Mon, Apr 6, 2009 at 7:37 AM, Andreas Pflug
> <pgadmin@pse-consulting.de <mailto:pgadmin@pse-consulting.de>> wrote:
>
>     Running 8.3.7, I have a warm standby configuration with a
>     archive_timeout of 10min.
>
>     It's obvious that there's a 10min period where data could be lost
>     if the
>     master fails and the warm standby server has to take over. What's not
>     obvious is that this is true even if the master server is shut down
>     regularly, because it will not write out a last log segment to the
>     archive. As a consequence, when doing a controlled failover (for
>     maintenance purposes or so) all data changed after the last
>     archive copy
>     will be lost.
>     IMHO this should be mentioned in the docs explicitly (I find it quite
>     surprising that data can be lost even if the system is shutdown
>     correctly), or better when shutting down the postmaster should
>     spit all
>     log segments containing all changes when archiving is on so the warm
>     standby server can catch up.
>
>
>
> You make an excellent point.  If you're looking for a way to mitigate
> this risk, run:
>
>     select pg_switch_xlog() ;
>
>    Before shutting down.
Sort of, unless some other user succeeds to commit a transaction after
pg_switch_xlog, and before the database ceases operation.

My "graceful failover" procedure now includes this workaround:
- shutdown server
- restart server with --listen_addresses='' to prevent other users to
connect (there are no local users on the server machine)
- pg_switch_xlog()
- shutdown finally
- let the warm server continue

Regards,
Andreas