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Re: Core team statement on replication in PostgreSQL

From: Hannu Krosing <hannu(at)krosing(dot)net>
To: Andreas 'ads' Scherbaum <adsmail(at)wars-nicht(dot)de>
Cc: Chris Browne <cbbrowne(at)acm(dot)org>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Core team statement on replication in PostgreSQL
Date: 2008-06-03 07:19:59
Message-ID: 1212477599.7267.3.camel@huvostro (view raw or flat)
Thread:
Lists: pgsql-advocacypgsql-hackers
On Mon, 2008-06-02 at 22:40 +0200, Andreas 'ads' Scherbaum wrote:
> On Mon, 02 Jun 2008 11:52:05 -0400 Chris Browne wrote:
> 
> > adsmail(at)wars-nicht(dot)de ("Andreas 'ads' Scherbaum") writes:
> > > On Thu, 29 May 2008 23:02:56 -0400 Andrew Dunstan wrote:
> > >
> > >> Well, yes, but you do know about archive_timeout, right? No need to wait 
> > >> 2 hours.
> > >
> > > Then you ship 16 MB binary stuff every 30 second or every minute but
> > > you only have some kbyte real data in the logfile. This must be taken
> > > into account, especially if you ship the logfile over the internet
> > > (means: no high-speed connection, maybe even pay-per-traffic) to the
> > > slave.
> > 
> > If you have that kind of scenario, then you have painted yourself into
> > a corner, and there isn't anything that can be done to extract you
> > from it.
> 
> You are misunderstanding something. It's perfectly possible that you
> have a low-traffic database with changes every now and then. But you
> have to copy a full 16 MB logfile every 30 seconds or every minute just
> to have the slave up-to-date.

To repeat my other post in this thread:

Actually we can already do better than ´╗┐file-by-file by using
pg_xlogfile_name_offset() which was added sometime in 2006. walmgr.py
from SkyTools package for example does this to get no more than a few
seconds failure window and it copies just the changed part of WAL to
slave.

pg_xlogfile_name_offset() was added just for this purpose - to enable
WAL shipping scripts to query, where inside the logfile current write
pointer is.

It is not synchronous, but it can be made very close, within subsecond
if you poll it frequently enough.

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



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