Re: recovery_target immediate timestamp - Mailing list pgsql-hackers

On 2020/11/12 6:00, Euler Taveira wrote:
> Hi,
> 
> While restoring a backup using recovery_target immediate, I noticed that there
> isn't a timestamp available.
> 
> LOG:  consistent recovery state reached at 0/A000100
> LOG:  recovery stopping after reaching consistency
> LOG:  pausing at the end of recovery
> HINT:  Execute pg_wal_replay_resume() to promote.
> LOG:  database system is ready to accept read only connections
> 
> if you decide to use one of the targets or just recover until the end of the
> WAL, you get a (last completed transaction) timestamp.
> 
> LOG:  redo done at 0/10FFEC38 system usage: CPU: user: 0.10 s, system: 0.05 s, elapsed: 1.65 s
> LOG:  last completed transaction was at log time 2020-11-11 17:27:31.715251-03
> LOG:  restored log file "000000010000000000000010" from archive
> cp: cannot stat '/a/pgarchive/00000002.history': No such file or directory
> LOG:  selected new timeline ID: 2
> LOG:  archive recovery complete
> cp: cannot stat '/a/pgarchive/00000001.history': No such file or directory
> LOG:  database system is ready to accept connections
> 
> I dig into the pg_waldump output to figure out the timestamp, however, the
> checkpoint timestamp isn't printed by pg_waldump. The checkpoint timestamp
> might be useful information at least when you set large values for a checkpoint
> or need to investigate a performance/corruption issue.
> 
> The first patch adds a new message that prints the latest completed checkpoint
> when the consistent state is reached.

I'm not sure how useful this information is in practice.

> It also exposes the checkpoint timestamp
> in debug messages.

              ereport(DEBUG1,
                      (errmsg("checkpoint record is at %X/%X",
                              (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
+            ereport(DEBUG1,
+                    (errmsg("checkpoint time is %s", str_time(checkPoint.time))));

The above first debug message displays the LSN of the checkpoint record.
OTOH, the second message displays the time when the checkpoint started
(not the time when checkpoint record was written at the end of checkpoint).
So isn't it confusing to display those inconsistent information together?

> 
> LOG:  consistent recovery state reached at 0/A000100
> DETAIL:  Last completed checkpoint was at log time 2020-11-11 17:31:50 -03.
> LOG:  recovery stopping after reaching consistency
> LOG:  pausing at the end of recovery
> HINT:  Execute pg_wal_replay_resume() to promote.
> LOG:  database system is ready to accept read only connections
> .
> .
> .
> DEBUG:  checkpoint record is at 0/A000060
> DEBUG:  checkpoint time is 2020-11-11 17:34:19 -03
> 
> The second patch provides the checkpoint timestamp in the pg_waldump output and
> also when you enable wal_debug parameter. The pg_waldump output looks like

+1

+#ifdef FRONTEND
+        strftime(checkpointstr, sizeof(checkpointstr), "%c", localtime(&time_tmp));
+#else
+        pg_strftime(checkpointstr, sizeof(checkpointstr), "%c", pg_localtime(&time_tmp, log_timezone));
+#endif

You can simplify the code by using timestamptz_to_str() here instead, like xact_desc_commit() does.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION