Re: pg_receivewal starting position

At Tue, 27 Jul 2021 07:50:39 +0200, Ronan Dunklau <ronan.dunklau@aiven.io> wrote in 
> Hello,
> 
> I've notived that pg_receivewal logic for deciding which LSN to start 
> streaming at consists of:
>   - looking up the latest WAL file in our destination folder, and resume from 
> here
>   - if there isn't, use the current flush location instead.
> 
> This behaviour surprised me when using it with a replication slot: I was 
> expecting it to start streaming at the last flushed location from the 
> replication slot instead. If you consider a backup tool which will take 
> pg_receivewal's output and transfer it somewhere else, using the replication 
> slot position would be the easiest way to ensure we don't miss WAL files.
> 
> Does that make sense ? 
> 
> I don't know if it should be the default, toggled by a command line flag, or if 
> we even should let the user provide a LSN.

*I* think it is completely reasonable (or at least convenient or less
astonishing) that pg_receivewal starts from the restart_lsn of the
replication slot to use.  The tool already decides the clean-start LSN
a bit unusual way. And it seems to me that proposed behavior can be
the default when -S is specified.

> I'd be happy to implement any of that if we agree.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center