Re: WAL consistency check facility

On Sat, Sep 10, 2016 at 8:33 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Sat, Sep 10, 2016 at 3:19 AM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)com> wrote:
>> On Fri, Sep 9, 2016 at 4:01 PM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
>>>>> - If WAL consistency check is enabled for a rmgrID, we always include
>>>>> the backup image in the WAL record.
>>>>
>>>> What happens if wal_consistency has different settings on a standby
>>>> and its master? If for example it is set to 'all' on the standby, and
>>>> 'none' on the master, or vice-versa, how do things react? An update of
>>>> this parameter should be WAL-logged, no?
>>>>
>>> It is possible to set wal_consistency to 'All' in master and any other
>>> values in standby. But, the scenario you mentioned will cause error in
>>> standby since it may not get the required backup image for wal
>>> consistency check. I think that user should be responsible to set
>>> this value correctly. We can improve the error message to make the
>>> user aware of the situation.
>>
>> Let's be careful here. You should as well consider things from the
>> angle that some parameter updates are WAL-logged as well, like
>> wal_level with the WAL record XLOG_PARAMETER_CHANGE.
>
> It seems entirely unnecessary for the master and the standby to agree
> here. I think what we need is two GUCs. One of them, which affects
> only the master, controls whether the validation information is
> including in the WAL, and the other, which affects only the standby,
> affects whether validation is performed when the necessary information
> is present.
>

I think from the clarity perspective, this option sounds good, but I
am slightly afraid that it might be inconvenient for users to set the
different values for these two parameters.

> Or maybe skip the second one and just decree that
> standbys will always validate if the necessary information is present.

Sounds like a better alternative and probably easier to configure for users.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com