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Re: [Pgcluster-general] PostgreSQL Documentation of High Availability

From: Markus Schiltknecht <markus(at)bluegap(dot)ch>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: a(dot)mitani(at)sra-europe(dot)com, pgsql-docs(at)postgresql(dot)org, pgcluster-general(at)pgfoundry(dot)org
Subject: Re: [Pgcluster-general] PostgreSQL Documentation of High Availability
Date: 2006-11-21 08:23:50
Message-ID: 4562B796.9070601@bluegap.ch (view raw or flat)
Thread:
Lists: pgsql-docs
Hi,

Bruce Momjian wrote:
> I feel the shared-* issue splits us up like master/slave and
> multi-master splits up

No, not quite. To sum up, I'd say the following combinations make sense:

sync, multi-master replication on shared-memory cluster (which is much 
like a super-computer. With shared memory distributing locks does not 
cost much - beside marketing, there is probably not much sense in 
calling this a cluster at all).

sync, multi-master replication on shared-disk cluster (where locks and 
memory-caches have to be synchronized. OracleRAC and PgCluster-II fit in 
here.)

(Probably running an async replication on a shared-disk cluster would 
make sense with MVCC and in some corner cases, but I don't see much 
benefits in that.)

sync, multi-master replication on shared-nothing cluster (where locks, 
caches and data needs to be synchronized over an interconnect. 
Postgres-R, PgCluster, PgPool)

(sync, single-master replication does not make much sense, because if 
you go sync at all, you could as well use the nodes which run in sync).

async, multi-master replication on shared-nothing cluster (i.e. Slony-I)

async, single-master replication on shared-nothing cluster (mainly for 
failover purpose, you mention solutions for that)


For me these categorizations are important and help a good deal to 
ensure what I'm talking about with somebody. The documentation is much 
more focused on individual solutions, sometimes avoiding to categorize 
them. I would love to get others opinions, but as not many others speak 
up, I just accept it that way.

> Yea, gets confusing.

Well, Oracle also does a good deal in making it confusing, IMO.

> Good point. I mentioned Oracle RAC only because it seems to be an
> industry standard, so by mentioning it, people know exactly what we are
> talking about.  

That's a point, even if I don't really know how much of an industry 
standard it is. But given how badly Oracle does in explaining basics of 
replication and clustering, I think it's not very beneficial.

> Is there a better way?  And people do ask for Oracle
> RAC, so in a way we are telling them we don't have something similar. 
> As sad as that is, it is true currently.

How far is PGCluster-II? Does it make sense to mention it? Can 
PGCluster-II be used with network filesystems like NFS, OCFS2 or the like?

> pgcluster is must closer to Oracle RAC, 

Why do you think so? Oracle RAC is mainly based on a shared disk 
cluster, where PGCluster bases on a shared nothing architecture. 
PGCluster-II seems closer to Oracle RAC, for me.

> but I haven't mentioned it
> because I am unsure where it is in terms of usability and stability.
> Comments?

Did you work on it since Toronto, Mitani-San?

Regards

Markus


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