Re: The behavior of CheckRequiredParameterValues()

On Wed, Mar 5, 2014 at 4:09 AM, Sawada Masahiko <sawada(dot)mshk(at)gmail(dot)com>wrote:

> Hi all,
>
> I had doubts regarding behavior of CheckRequiredParameterValues() function.
>
> I could not start standby server which is created by pg_basebackup
> with following scenario.
> 1. Start the master server with 'wal_level = archve' , 'hot_standby =
> on' and other settings of replication.
> 2. Create the standby server from the master server by using pg_basebackup.
> 3. Change the wal_level value of both master and standby server to
> 'hot_standby'.
> 4. Restarting the master server.
> 5. Starting the standby server.
>
> In #5, I got following error even if I set wal_level to 'hot_standby'.
>
> FATAL: hot standby is not possible because wal_level was not set to
> "hot_standby" or higher on the master server
>
> I tried to investigate this behaviour.
> Currently CheckRequiredParameterValues() function uses wal_level value
> which is got from ControlFile when comparing between wal_level and
> WAL_LEVEL_HOT_STANDBY as following code.
>
> xlog.c:6177
> if (ControlFile->wal_level < WAL_LEVEL_HOT_STANDBY)
> ereport(ERROR,
> (errmsg("hot standby is not possible because wal_level was not
>
> So we have to start and stop standby server with changed
> wal_level(i.g., hot_standby) if we want to enable hot standby.
> In this case, I think that the standby server didn't need to confirm
> wal_level value of ControlFile.
> I think that it should confirm value which is written in postgreql.conf.
>

The snapshot of running transaction information is written to WAL only when
the wal_level is set to 'hot_standby'.
This information is required on the standby side to recreate the running
transactions.

Regards,
Hari Babu
Fujitsu Australia