Re: Switching timeline over streaming replication

> From: Heikki Linnakangas [mailto:hlinnakangas@vmware.com]
> Sent: Friday, December 07, 2012 9:22 PM
> To: Amit Kapila
> Cc: 'PostgreSQL-development'; 'Thom Brown'
> Subject: Re: [HACKERS] Switching timeline over streaming replication
> 
> On 06.12.2012 15:39, Amit Kapila wrote:
> > On Thursday, December 06, 2012 12:53 AM Heikki Linnakangas wrote:
> >> On 05.12.2012 14:32, Amit Kapila wrote:
> >>> On Tuesday, December 04, 2012 10:01 PM Heikki Linnakangas wrote:
> >>>> After some diversions to fix bugs and refactor existing code, I've
> >>>> committed a couple of small parts of this patch, which just add
> >>>> some sanity checks to notice incorrect PITR scenarios. Here's a new
> >>>> version of the main patch based on current HEAD.
> >>>
> >>> After testing with the new patch, the following problems are
> observed.
> >>>
> >>> Defect - 1:
> >>>
> >>>       1. start primary A
> >>>       2. start standby B following A
> >>>       3. start cascade standby C following B.
> >>>       4. start another standby D following C.
> >>>       5. Promote standby B.
> >>>       6. After successful time line switch in cascade standby C&
> D,
> >> stop D.
> >>>       7. Restart D, Startup is successful and connecting to standby
> C.
> >>>       8. Stop C.
> >>>       9. Restart C, startup is failing.
> >>
> >> Ok, the error I get in that scenario is:
> >>
> >> C 2012-12-05 19:55:43.840 EET 9283 FATAL:  requested timeline 2 does
> >> not contain minimum recovery point 0/3023F08 on timeline 1 C
> >> 2012-12-05
> >> 19:55:43.841 EET 9282 LOG:  startup process (PID 9283) exited with
> >> exit code 1 C 2012-12-05 19:55:43.841 EET 9282 LOG:  aborting startup
> >> due to startup process failure
> >>
> >
> >>
> Well, it seems wrong for the control file to contain a situation like
> this:
> 
> pg_control version number:            932
> Catalog version number:               201211281
> Database system identifier:           5819228770976387006
> Database cluster state:               shut down in recovery
> pg_control last modified:             pe  7. joulukuuta 2012 17.39.57
> Latest checkpoint location:           0/3023EA8
> Prior checkpoint location:            0/2000060
> Latest checkpoint's REDO location:    0/3023EA8
> Latest checkpoint's REDO WAL file:    000000020000000000000003
> Latest checkpoint's TimeLineID:       2
> ...
> Time of latest checkpoint:            pe  7. joulukuuta 2012 17.39.49
> Min recovery ending location:         0/3023F08
> Min recovery ending loc's timeline:   1
> 
> Note the latest checkpoint location and its TimelineID, and compare them
> with the min recovery ending location. The min recovery ending location
> is ahead of latest checkpoint's location; the min recovery ending
> location actually points to the end of the checkpoint record. But how
> come the min recovery ending location's timeline is 1, while the
> checkpoint record's timeline is 2.
> 
> Now maybe that would happen to work if remove the sanity check, but it
> still seems horribly confusing. I'm afraid that discrepancy will come
> back to haunt us later if we leave it like that. So I'd like to fix
> that.
> 
> Mulling over this for some more, I propose the attached patch. With the
> patch, we peek into the checkpoint record, and actually perform the
> timeline switch (by changing ThisTimeLineID) before replaying it. That
> way the checkpoint record is really considered to be on the new timeline
> for all purposes. At the moment, the only difference that makes in
> practice is that we set replayEndTLI, and thus minRecoveryPointTLI, to
> the new TLI, but it feels logically more correct to do it that way.

This has fixed both the problems reported in below link:
http://archives.postgresql.org/pgsql-hackers/2012-12/msg00267.php

The code is also fine.

With Regards,
Amit Kapila.