Re: Avoiding timeline generation

On 25.03.2011 03:00, Daniel Farina wrote:
> Here is the mechanism: I want to author a recovery.conf to perform
> some amount of restore_command or streaming replication based
> recovery, but I do *not* want to generate a new timeline. Rather, I
> want to stay in hot standby mode to allow read-only connections.

That's exactly what the standby mode is for. Add "standby_mode=on" to
recovery.conf, and the server will do exactly that.

Perhaps the documentation is not clear on this. Any suggestions on how
to improve that?

> InArchiveRecovery gets set to "true" as soon as
> readRecoveryCommandFile completes basically normally, and it looks as
> though that will ensure we will get a new timeline. If one tries a
> bizarre hack, like ensuring the restore_command does not terminate,
> one never finishes recovery

That's what pg_standby does. That was the only option before
standby_mode was introduced, in version 9.0, although we didn't have hot
standby until 9.0 either.

> -- as one may expect -- and one cannot
> connect to the server -- which one may not expect is necessarily the
> case presuming hot standby, if the server was terminated cleanly.

That's not true. As long as you enable hot standby, the server will
accept connections while restore command is running.

> The things I want to do with the ability to suppress a new timeline:
>
> * Offline WAL application -- I want to be able to bring up a second
> server, perform some amount of point in time recovery, and then stop
> and archive. It would be nice to support read-only queries in this
> case to test the recovered database. The goal of this is to reduce
> recovery time in a disaster scenario without tying up resources on a
> live server.

Yep, that can be done with standby_mode=on.

> * The ability to quiesce a system by bringing it into read-only state
> that generates no new WAL while still being able to ship old WAL.

In theory it should be possible to stop a server, put it into hot
standby mode by creating a recovery.conf file, and restart, but it won't
try ship the old WAL after that. When you stop a server it will try to
archive all existing WAL before exiting, though.

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