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Re: High Avaliable in the PostgreSQL.

From: Andrew Sullivan <ajs(at)crankycanuck(dot)ca>
To: pgsql-admin(at)postgresql(dot)org
Subject: Re: High Avaliable in the PostgreSQL.
Date: 2007-05-31 14:43:12
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-admin
On Thu, May 31, 2007 at 05:30:20PM +1200, adey wrote:
> HP was providing CA (Continuous Access) software that was claimed to provide
> WAN SAN replication by repeating IO in exactly the sequence it was generated
> on the master, to the slave. 

The CA stuff, or anything else built on FCIP, is pretty cool, but I'd
worry a little about latency.  Certainly your costs are going to be
high on transit, but if the data and uptime are worth it, you could
do it.  The big issue here is the same as in any other
failover-to-other-node case -- you have to be _super_ sure that the
failing master writer is dead and disconnected, and across a wide
area, this is going to be very tough to do.  I think you'd have to
spend a fair amount of time doing risk analysis of conflicting
commits happening in both sites (if the WAN link goes down because,
say, some genius decides to run a backhoe through the fiber going out
of your "primary" city, you could find your secondary site decides to
promote itself.  But the primary site might still have transactions
in flight.)

> system. I'm not sure this would be sufficient for 99.9% uptime though, as
> there would be some startup requirements on the slave.

Right, and the requirement was actually upped to 99.99%, which is
approximately 1 hour of allowable downtime a year.  That is a very
high bar.


Andrew Sullivan  | ajs(at)crankycanuck(dot)ca
Users never remark, "Wow, this software may be buggy and hard 
to use, but at least there is a lot of code underneath."
		--Damien Katz

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