Re: Streaming replication and WAL archive interactions

On Tue, Apr 21, 2015 at 6:55 AM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> On 04/21/2015 12:04 PM, Michael Paquier wrote:
>> On Tue, Apr 21, 2015 at 4:38 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
>> wrote:
>>> Note that even though we don't archive the partial last segment on the
>>> previous timeline, the same WAL is copied to the first segment on the new
>>> timeline. So the WAL isn't lost.
>>
>> But if the failed master has archived those segments safely, we may need
>> them, no? I am not sure we can ignore a user who would want to do a PITR
>> with recovery_target_timeline pointing to the one of the failed master.
>
> I think it would be acceptable. If you want to maintain an up-to-the-second
> archive, you can use pg_receivexlog. Mind you, if the standby wasn't
> promoted, the partial segment would not be present in the archive anyway.
> And you can copy the WAL segment manually from 0000000200000000000000XX to
> pg_xlog/0000000100000000000000XX before starting PITR.
>
> Another thought is that we could archive the partial file, but with a
> different name to avoid confusing it with the full segment. For example, we
> could archive a partial 000000010000000000000012 segment as
> "000000020000000000000012.00000128.partial", where 00000128 indicates how
> far that file is valid (this naming is similar to how the backup history
> files are named). Recovery wouldn't automatically pick up those files, but
> the DBA could easily copy the partial file into pg_xlog with the full
> segment's name, if he wants to do PITR to that piece of WAL.

So, suppose you A replicating to B (via an archive) replicating to C
(via a separate archive); A dies, B is promoted. It sounds to me like
today this will work and with your proposed change it will require
manual intervention. I don't think that's OK.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company