Re: What are the differences between Real Application Clusters, Guard I and Guard II?

Real Application Clusters used to be called Parallel Server. You can think of it as a number of physically separate computers each containing a synchronised copy of the database. In my opinion, the main advantages are redundancy and scalability. If one of the nodes in your cluster goes down, the others continue working. If your cluster starts getting overloaded, you can add more nodes.

The main disadvantage is that RAC depends on the clustering solutions offered by your server manufacturer. This can mean that the nodes in your cluster need to be physically close together. Hence if you get a disaster like a fire in your data centre, the entire cluster can be destroyed. Redundant nodes won't save you. I've heard that some operating systems (VMS?) can implement clustering over a wide area. I'd be interested to hear from anybody who knows about this. Also, I've seen presentations by Compaq showing a wide area clustering technique linking nodes with a fibre that can be thousands of meters long. Not much use though unless you are in a campus environment where you have access to cable ducts to string the fibre between buildings.

Another problem is that some clusters consist of a number of nodes connected to a communal disk array or hub. Here you have redundancy among nodes but the disk array or hub still forms a single point of failure. You can mitigate this to some extent using RAID arrays but, in a catastrophe like 9/11 or the Canary Wharf bomb, you still lose the lot.

I think data guard has always been called data guard but I believe there is a similar product called Fail Safe that is only available for Windows platforms. I've never used data guard but my understanding is that you have a primary node and one or more secondary nodes. As redo logs are generated they are copied to the secondary nodes. The secondaries run in continuous backup recovery mode and apply the archived redo logs as they are received. Interconnection can be achieved using an ordinary wan link so your nodes can be widely separated. Unlike RAC, this provides protection against disasters. If your primary data centre burns down, your secondary nodes can be hundreds of miles away so they will still be available.

The disadvantage with data guard is that your databases are not synchronised. Some delays are inevitable while the redo log is being built up and then transmitted. Another option, replication, allows changes to be committed in smaller chunks over a WAN so your databases are out of sync for shorter periods. Replication can be tricky to implement though.

Best wishes,

Richard

"Eric" <eric32_at_hotmail.com> wrote in message news:AyN6b.437$fC5.147403_at_news20.bellglobal.com...
> Hi all,
>
> I have some confusion about Oracle 9i's new components.
> What are the differences between Real Application Clusters, Guard I and
> Guard II? What are their corresponding components names in the previous
> releases?
>
> Thanks!
>
> Eric
>
>
>
>
Received on Sun Sep 07 2003 - 18:19:45 CDT