Re: [HACKERS] Why restore_command is called for existing files in pg_xlog?

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От Jeff Janes
Тема Re: [HACKERS] Why restore_command is called for existing files in pg_xlog?
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Msg-id CAMkU=1wR7b=orKKkwgjiJFFHd54S+c64Nr3v_oEZP6QT+2eeUw@mail.gmail.com
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Ответ на Re: [HACKERS] Why restore_command is called for existing files inpg_xlog?  (Alex Kliukin <alexk@hintbits.com>)
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On Mon, Jun 12, 2017 at 5:25 AM, Alex Kliukin <alexk@hintbits.com> wrote:
On Fri, Jun 2, 2017, at 11:51 AM, Alexander Kukushkin wrote:
Hello hackers,
There is one strange and awful thing I don't understand about restore_command: it is always being called for every single WAL segment postgres wants to apply (even if such segment already exists in pg_xlog) until replica start streaming from the master.

The real problem this question is related to is being unable to bring a former master, demoted after a crash, online, since the WAL segments required to get it to the consistent state were not archived while it was still a master, and local segments in pg_xlog are ignored when a restore_command is defined. The other replicas wouldn't be good candidates for promotion as well, as they were way behind the master (because the last N WAL segments were not archived and streaming replication had a few seconds delay).

I don't really understand the problem.  If the other replicas are not candidates for promotion, than why was the master ever "demoted" in the first place?  It should just go through normal crash recovery, not PITR recovery, and therefore will read the files from pg_xlog just fine.

If you already promoted one of the replicas and accepted data changes into it, and now are thinking that was not a good idea, then there is no off the shelf automatic way to merge together the two systems.  You have do a manual inspection of the differences.  To do that, you would start by starting up (a copy of) the crashed master, using normal crash recovery, not PITR.

 

Is this a correct list for such questions, or would it be more appropriate to ask elsewhere (i.e. pgsql-bugs?)

Probably more appropriate for pgsql-general or pgsql-admin.

Cheers,

Jeff

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