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Date: Fri, 4 Dec 2015 23:45:58 +0900
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Subject: Re: [DOCS] max_worker_processes on the standby
From: Fujii Masao <masao.fujii@gmail.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Petr Jelinek <petr@2ndquadrant.com>,
 oonishitk@nttdata.co.jp, pgsql-docs <pgsql-docs@postgresql.org>,
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On Tue, Nov 17, 2015 at 6:43 AM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> I paraphrase Fujii Masao, who wrote:
>
>> 1. Start the master and standby servers with track_commit_timestamp enabled.
>> 2. Disable track_commit_timestamp in the master and restart the master server.
>> 3. Run checkpoint in the master.
>> 4. Run restartpoint in the standby after the checkpoint WAL record generated
>> 5. Restart the standby server.
>> 6. Enable track_commit_timestamp in the master and restart the master server.
>> 7. Disable track_commit_timestamp in the master and restart the master server.
>
>> What I think strange is that pg_last_committed_xact() behaves differently
>> in #2, #5, and #7 though the settings of track_commit_timestamp are same
>> in both servers, i.e., it's disabled in the master but enabled in the standby.
>
> Interesting, thanks.  You're right that this behaves oddly.
>
> I think in order to fix these two points (#5 and #7), we need to make
> the standby not honour the GUC at all, i.e. only heed what the master
> setting is.
>
>> 8. Promote the standby server to new master.
>>     Since committs is still inactive even after the promotion,
>>     pg_last_committed_xact() keeps causing an ERROR though
>>     track_commit_timestamp is on.
>>
>> I was thinking that whether committs is active or not should depend on
>> the setting of track_commit_timestamp *after* the promotion.
>> The behavior in #8 looked strange.
>
> To fix this problem, we can re-run StartupCommitTs() after recovery is
> done, this time making sure to honour the GUC setting.
>
> I haven't tried the scenarios we fixed with the previous round of
> patching, but this patch seems to close the problems just reported.
> (My next step will be looking over the recovery test framework by
> Michael et al, so that I can apply a few tests for this stuff.)
> In the meantime, if you can look this over I would appreciate it.

Sorry for not reviewing the patch before you push it...

In HEAD, I ran very simple test case:

1. enable track_commit_timestamp
2. start the server
3. run some transactions
4. execute pg_last_committed_xact() -- returns non-null values
5. shutdown the server with immdiate mode
6. restart the server -- crash recovery happens
7. execute pg_last_committed_xact()

The last call of pg_last_committed_xact() returns NULL values, which means
that the xid and timestamp information of the last committed transaction
disappeared by crash recovery. Isn't this a bug?

Regards,

-- 
Fujii Masao


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