Date: Tue, 21 Dec 2010 05:03:44 +0100
To: Scott Marlowe <scott.marlowe@gmail.com>
From: snoop@email.it
Cc: pgsql-admin@postgresql.org
Reply-to: snoop@email.it
Subject: Re: PostgreSQL in Shared Disk Failover mode on FreeBSD+CARP+RAIDZ
Message-ID: <0d36ce42c9d5fd79b7122ce8c7e17218@213.152.156.147>
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 > On Mon, Dec 20, 2010 at 8:03 PM,  <snoop@email.it> wrote:
> >> On Mon, Dec 20, 2010 at 6:23 PM,  <snoop@email.it> wrote:
> > Well, I'd prefer a big single storage instead of "too many spinning
disks"
> > around mainly for maintenance reasons, to avoid replication and too much
> > network traffic.
> 
> Sure, but that then makes your single point of failure your storage
> system.  With no replication the best you can hope for if the storage
> array fails completely is to revert to a recent backup, since without
> any streaming replication you'll be missing tons of data if you've got
> many updates.  If your database is mostly static or the changes can be
> recreated from other sources that's not so bad.  However, if you have
> one storage array and it has a carastrophic hardware failure and goes
> down and stay down, then you'll need something else to hold the db
> while you wait for parts etc.

I'd use a filesystem replication solution like DRBD to avoid one disk array
being a single point of failure.

> 
> >> > - I don't like the idea of having fixed size (16 megs regardless of
the
> >> > committed transaction number!) WAL logs often "shipped" from one node
to
> >> > another endangering my network performance (asynchronous replication)
> 
> (P.s. that was pre 9.0 PITR...)
> 
> >> Streaming replication in 9.0 doesn't really work that way, so you
> >> could use that now with a hot standby ready to be failed over to as
> >> needed.
> >
> > Mmm, so I can use an hot standby setup without any need for replication
> > (same data dir) and no need for STONITH?
> > Sorry if my questions sound trivial to you but my experience with
PostgreSQL
> > is quite limited and this would be my first "more complex" configuration
and
> > I'm trying to figure out the best way to go. Unfortunately it's not that
> > easy to figure it out going through the documentation only.
> 
> No no.  With streaming replication the master streams changes to the
> slave(s) in real time, not via copying WAL files like in previous PITR
> replication.  No need for STONITH and / or fencing since the master
> database writes to the slave database.  Failover would be provided by
> whatever monitoring script you want to throw at the system, maybe with
> pgpool, pgbouncer, or even CARP if you wanted (I'm no fan of carp, had
> a lot of problems with it and CISCO switches a while back).  Cut off
> the main server,  put slave server into recovery, when recovery
> finishes and it's up and running, change the IP in the app config,
> bounce app and keep going.  With slony you'd do something similar, but
> use slonik commands to promote the first slave to the master where
> you'd bring up the slave in recovery mode in the streaming replication
> method.
> 
> All of that assumes two machines with their own storage. (technically
> you could put it all on the same big array in different directories).
> 
> If you want to share the same data dir then you HAVE to make sure that
> only one machine at a time can ever open that directory and start
> postgresql there.  Two postmasters on the same data dir are instant
> death.

OK, I still have to study a lot this technology before prior to keep on but
now I know where to look and how.
Thank you very much for your time! I really appreciate that. 
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