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Re: Core team statement on replication in PostgreSQL

From: Hannu Krosing <hannu(at)krosing(dot)net>
To: Mathias Brossard <mathias(dot)brossard(at)opentrust(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Core team statement on replication in PostgreSQL
Date: 2008-06-02 17:37:59
Message-ID: 1212428279.8183.5.camel@huvostro (view raw or flat)
Thread:
Lists: pgsql-advocacypgsql-hackers
On Thu, 2008-05-29 at 23:37 +0200, Mathias Brossard wrote:

> I pointed out that the NTT solution is synchronous because Tom said in 
> the first part of his email that:
> 
>  > In practice, simple asynchronous single-master-multiple-slave
>  > replication covers a respectable fraction of use cases, so we have
>  > concluded that we should allow such a feature to be included in the
>  > core project.
> 
> ... and yet "the most appropriate base technology for this" is 
> synchronous and maybe I should have also pointed out in my previous mail 
> is that it doesn't support multiple slaves.

I don't think that you need too many slaves in sync mode.

Probably 1-st slave sync and others async from there on will be good
enough.

> Also, as other have pointed out there are different interpretations of 
> "synchronous" depending on wether the WAL data has reached the other end 
> of the network connection, a safe disk checkpoint or the slave DB itself.

Probably all DRBD-s levels ( A) data sent to network, B) data received,
C) data written to disk) should be supported + C1) data replayed in
slave DB. C1 meaning that it can be done in parallel with C)

Then each DBA can set it up depending on what he trusts - network,
slave's power supply or slaves' disk.

Also, the case of slave failure should be addressed. I don't think that
the best solution is halting all ops on master if slave/network fails.

Maybe we should allow also a setup with 2-3 slaves, where operations can
continue when at least 1 slave is "syncing" ?

--------------
Hannu


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