Re: Fetching timeline during recovery

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Jehan-Guillaume de Rorthais <jgdr(at)dalibo(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Fetching timeline during recovery
Date: 2019-07-23 18:59:00
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> 23 июля 2019 г., в 21:05, Jehan-Guillaume de Rorthais <jgdr(at)dalibo(dot)com> написал(а):
> Fetching the timeline from a standby could be useful in various situation.
> Either for backup tools [1] or failover tools during some kind of election
> process.
That backup tool is reading timeline from pg_control_checkpoint(). And formats WAL file name itself when necessary.

> Please, find in attachment a first trivial patch to support pg_walfile_name()
> and pg_walfile_name_offset() on a standby.

You just cannot format WAL file name for LSN when timeline changed. Because there are at least three WALs for that point: previous, new and partial. However, reading TLI from checkpoint seems safe for backup purposes.
The only reason for WAL-G to read that timeline is to mark backup invalid: if it's name is base_00000001XXXXXXXXYY00000YY and timeline change happens, it should be named base_00000002XXXXXXXXYY00000YY (consistency point is not on TLI 2), but WAL-G cannot rename backup during backup-push.

Hope this information is useful. Thanks!

Best regards, Andrey Borodin.


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