Re: [GENERAL] Why does this hot standy archive_command work

----- Original Message -----
> From: "Jerry Sievers" <gsievers19@comcast.net>
> To: "Steve Atkins" <steve@blighty.com>
> Cc: "pgsql-general" <pgsql-general@postgresql.org>
> Sent: Monday, January 23, 2017 12:52:46 PM
> Subject: Re: [GENERAL] Why does this hot standy archive_command work
>
> Steve Atkins <steve@blighty.com> writes:
>
> >> On Jan 20, 2017, at 7:03 PM, btober@computer.org <btober@broadstripe.net>
> >> wrote:
> >>
> >> While learning a bit about basic hot standby configuration, I was
> >> reviewing an article that used these parameters
> >>
> >> wal_level = 'hot_standby'
> >> archive_mode = on
> >> archive_command = 'cd .'
>
> That's just a very silly way of making archive_command a true no-op...
>
> I would have set it to '/bin/true'


Roger that.

Also, I guess I should have maybe been more clear what I meant by "it works" ... the hot standby replication works,
whichis really what I was focusing on.  

I think I might have previously been under the mis-impression that archiving the WAL to ... somewhere ... was required
forhot stand-by, so the no-op threw me since the stand-by configuration does not seem to specify where to get the
primaryWAL from. It seems like what is happening here is that since the primary archive, as you point out, is no-op,
andthe standby server knows where to just grab the WAL directly.  

Thanks for replying!


>
>
>
> >> max_wal_senders = 1
> >> hot_standby = on
> >>
> >>
> >> How or why that particular archive_command actually works (... and it does
> >> ... I tried it ...) is not clear to me based on reading of the Postgresql
> >> documentation on this topic. I would have expected to see an actual copy
> >> or rsync command, as described in the fine manual at section 25.3.1.
> >> "Setting Up WAL Archiving"
> >>
> >> The entire example appears at
> >>
> >>
https://www.digitalocean.com/community/tutorials/how-to-set-up-master-slave-replication-on-postgresql-on-an-ubuntu-12-04-vps
> >>
> >> Can anyone enlighten on this topic, or provide a link to an existing
> >> explanation?
> >
> > It's not archiving logs at all, instead relying on streaming them directly
> > to the slave.
> >
> > Changing archive_mode requires a server restart, while changing
> > archive_command from a command that does nothing, successfully, to a
> > command that actually archives logs just requires a reload. So this lets
> > you enable archiving without halting the server by changing the command.
> >
> > Or that's how I vaguely recall it working some years ago. Things may have
> > changed now - you're following a very old tutorial.
> >
> > Cheers,
> >   Steve
>
> --
> Jerry Sievers
> Postgres DBA/Development Consulting
> e: postgres.consulting@comcast.net
> p: 312.241.7800
>
>
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