Re: history file on replica and double switchover - Mailing list pgsql-hackers

On 2020/09/25 8:15, David Zhang wrote:
> Hi,
> 
> My understanding is that the "archiver" won't even start if "archive_mode = on" has been set on a "replica".
Therefore,either (XLogArchiveMode == ARCHIVE_MODE_ALWAYS) or (XLogArchiveMode != ARCHIVE_MODE_OFF) will produce the
sameresult.
 

Yes, the archiver isn't invoked in the standby when archive_mode=on.
But, with the original patch, walreceiver creates .ready archive status file
even when archive_mode=on and no archiver is running. This causes
the history file to be archived after the standby is promoted and
the archiver is invoked.

With my patch, walreceiver creates .ready archive status for the history file
only when archive_mode=always, like it does for the streamed WAL files.
This is the difference between those two patches. To prevent the streamed
timeline history file from being archived, IMO we should use the condition
archive_mode==always in the walreceiver.


> 
> Please see how the "archiver" is started in src/backend/postmaster/postmaster.c
> 
> 5227                 /*
> 5228                  * Start the archiver if we're responsible for (re-)archiving received
> 5229                  * files.
> 5230                  */
> 5231                 Assert(PgArchPID == 0);
> 5232                 if (XLogArchivingAlways())
> 5233                         PgArchPID = pgarch_start();
> 
> I did run the nice script "double_switchover.sh" using either "always" or "on" on patch v1 and v2. They all generate
thesame results below. In other words, whether history file (00000003.history) is archived or not depends on
"archive_mode"settings.
 
> 
> echo "archive_mode = always" >> ${PGDATA_NODE2}/postgresql.auto.conf
> 
> $ ls -l archive
> -rw------- 1 david david 16777216 Sep 24 14:40 000000010000000000000002
> ... ...
> -rw------- 1 david david 16777216 Sep 24 14:40 00000002000000000000000A
> -rw------- 1 david david       41 Sep 24 14:40 00000002.history
> -rw------- 1 david david       83 Sep 24 14:40 00000003.history
> 
> echo "archive_mode = on" >> ${PGDATA_NODE2}/postgresql.auto.conf
> 
> $ ls -l archive
> -rw------- 1 david david 16777216 Sep 24 14:47 000000010000000000000002
> ... ...
> -rw------- 1 david david 16777216 Sep 24 14:47 00000002000000000000000A
> -rw------- 1 david david       41 Sep 24 14:47 00000002.history
> 
> 
> Personally, I prefer patch v2 since it allies to the statement here,
https://www.postgresql.org/docs/13/warm-standby.html#CONTINUOUS-ARCHIVING-IN-STANDBY
> 
> "If archive_mode is set to on, the archiver is not enabled during recovery or standby mode. If the standby server is
promoted,it will start archiving after the promotion, but will not archive any WAL it did not generate itself."
 
> 
> By the way, I think the last part of the sentence should be changed to something like below:
> 
> "but will not archive any WAL, which was not generated by itself."

I'm not sure if this change is an improvement or not. But if we apply
the patch I proposed, maybe we should mention also history file here.
For example, "but will not archive any WAL or timeline history files
that it did not generate itself".

Regards,

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