[Linux-cluster] Load balancing clustered services

Mon Aug 18 13:22:54 UTC 2008

Both the RHCS page and your assessment are correct.  Keep in mind that
RHCS / GFS provide the host framework for applications to leverage for
high availability and/or scalability  -- simply installing and running
them alone are not enough.

What you use for hardware, what your application is capable of doing
within this environment, and its IMPLEMENTATION determine whether you
are attempting to achieve high availability and/or scalability.  At the
very least, you will have a better solution in place than running a
single monolithic server.

A simple example, if you run a monolithic database instance, and simply
want fail-over to another node, the resource group manager's policy for
that clustered services can do this for you -- without any manual
intervention -- such as moving its IP and disk resources and restarting
the database.  That IS THE BEST availability you can get out of such a
design.  And this does nothing to increase scalability.

But expand this use-case by implementing a database that was built for
high availability -- such as Cache ECP or Oracle RAC -- then such an
outage on one node (planned or unplanned) will be managed by RHCS / GFS
architecture to provide for 100% uptime.  But, you also get scalability
as a positive outcome from this same infrastructure AND implementation.

We are using RHCS / GFS to manage a Cache ECP environment.  The
production application / database is not split yet into multiple tiers,
but it is shadowed for quick fail-over.  Until it is broken up over
several servers, we will never achieve ~100% uptime ... there will
always be that downtime during service transitioning, planned or
unplanned.

We are also using RHCS only to front-end Peoplesoft (BEA WebLogic) for
high-availability, but implemented as an active-active server pair.
True, it also serves for scalability, even though a single server can
easily handle our load.  But if something happens to one server (runaway
process(es) from a bad script, bad application, etc.), we can shut it
down without interrupting service.

Hope this helps.

________________________________________________________________________
Robert Hurst, Sr. Caché Administrator
Beth Israel Deaconess Medical Center
1135 Tremont Street, REN-7
Boston, Massachusetts   02120-2140
617-754-8754 ∙ Fax: 617-754-8730 ∙ Cell: 401-787-3154
Any technology distinguishable from magic is insufficiently advanced.

On Sun, 2008-08-17 at 17:03 -0400, Jeff Sturm wrote:

> Greetings,
>  
> The Red Hat Cluster Suite page says the following:
> 
> 
>  "For applications that require maximum uptime, a Red Hat Enterprise
> Linux cluster with Red Hat Cluster Suite is the answer. Specifically
> designed for Red Hat Enterprise Linux, Red Hat Cluster Suite provides
> two distinct types of clustering:
> 
>     * Application/Service Failover - Create n-node server clusters for
> failover of key applications and services
>     * IP Load Balancing - Load balance incoming IP network requests
> across a farm of servers"
> 
> 
> The implication seems to be that the first type addresses high
> availability, and the second scalability.  What is the optimal way to
> get both?
> 
> Please understand that I am already a user of GFS and LVS.  I'm asking
> the question because the two seemingly have nothing in common.  For
> example, cman knows about cluster membership and can immediately react
> when a node leaves the cluster or is fenced.  On the other hand, LVS
> (together with either piranha or ldirectord) keeps a list of real
> servers, periodically checking each and removing any found to be
> unresponsive.
> 
> It seems like there are a couple drawbacks to this bifurcated design:
> 
> - once cman realizes a node has left the cluster, there is a delay
> before ipvs updates its configuration, during which user requests can be
> routed to a dead server
> - two distinct sets of cluster configurations have to be maintained
> 
> Am I misunderstanding something fundamental, or is that the way it is?
> 
> 
> -Jeff
> 
> --
> Linux-cluster mailing list
> Linux-cluster at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-cluster
> 
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