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'



>> 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
basedon 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
haltingthe 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
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