Re: Fetching timeline during recovery

On 7/23/19 2:59 PM, Andrey Borodin wrote:
>
>> 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.

We do the same [1].

>> 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.

Naming considerations aside, I don't think that a timeline switch during
a standby backup is a good idea, mostly because it is (currently) not
tested. We don't allow it in pgBackRest.

[1]
https://github.com/pgbackrest/pgbackrest/blob/release/2.15.1/lib/pgBackRest/Db.pm#L1008

--
-David
david(at)pgmasters(dot)net