5 Replies Latest reply: May 2, 2011 8:58 AM by Levi Pereira RSS

    Are 3 IPs required for a SCAN in 11.2?

    dba69
      I've read the docs on setting up SCAN for 11.2. Oracle recommends 3 IPs but doesn't say what may happen if you only use one. We have a limited number of IPs and our admin guys want us to use only one. If we do use only 1 IP for the SCAN, what sort of trouble might this cause? Is there anything else we should do if we use only 1 IP?
        • 1. Re: Are 3 IPs required for a SCAN in 11.2?
          Sebastian Solbach -Dba Community-Oracle
          Hi,

          if you only use one, then you only have 1 SCAN VIP and 1 SCAN listener. One scan vip and listener can only be active on one node at a time, and will failover to another.
          So there are some things to know
          1.) What happens, if you only have one listener instead of 3, regarding connections:
          Connections will not be distributed on multiple SCAN listener, but always end up on the same. So be carefull, that this is not a bottleneck.
          Connection storms on 1 listener is simply more likely to happen, than if you have 3. But this can be avoided, in setting up the clients in a manner they don't open 100rds of connections at the same time - and with some listener.ora parameters.
          2.) What happens, if the listener is not available (because it's in the process of failing over):
          With 3 scans, the client will simply switch to the next one and get a connection. With only 1 scan, the client will get an error (No listener).
          So for the time of the failover with only 1 scan, there will be unsuccessful connection attempts. However relocation of the SCAN VIP and listener is normally quite fast...
          However the listener registration by the database (from PMON) is only done every 60 seconds. So if listener is up (after failover), but no service registered (yet), the clients mayl recieve an error (No service).
          3.) What happens, if for any reason this one scan listener fails and cannot be restarted.
          While very unlikely to happen, because the clusterware tries to avoid this, this would result in a complete "inavailability" of the cluster. With 3 scans, while not good that one listener isn't running, it does not matter.

          Hope that helps,

          Regards
          Sebastian
          • 2. Re: Are 3 IPs required for a SCAN in 11.2?
            dba69
            Thanks, that confirms my initial thought.

            Rather than have the SCAN configured with new/different IPs, wouldn't it work just as well if it was configured with the VIPs or pubic IPs of the nodes in the cluster?
            • 3. Re: Are 3 IPs required for a SCAN in 11.2?
              Levi Pereira
              Hi,
              Rather than have the SCAN configured with new/different IPs, wouldn't it work just as well if it was configured with the VIPs or pubic IPs of the nodes in the cluster?
              Is it mandatory to use SCAN?
              No, but it's highly recommended to use SCAN unless there's strong business reason preventing it from being used.

              Is it supported to remove SCAN?
              SCAN is an elementary part of 11gR2 Grid Infrastructure, it's not supported to remove SCAN. So, you must use an IP at least.



              SCAN IP is a new "layer" network (oracle) with high availability that allows you to change the characteristics of your cluster (i.e add/remove nodes) without the need to make configuration changes in their clients "Grid concept".

              The IP SCAN and VIP have different goals, therefore working together.
              The VIP IP provides high availability at the level of the Cluster.
              The SCAN IP provides high availability at the level of the Clients.

              No matter if we are adding or removing nodes in a cluster or if the nodes are active or inactive, SCAN IP is always available for clients (Managing Connections-Failover) since one node of cluster is active.

              Why not use VIP or Public ?
              Because for us to have the concept of GRID we must configure the feature only one time and it will self manage and make cluster transparent to yours clients.
              If you add or remove an NODE it's require to modify the connections of yours all clients (VIP/PUBLIC IP) you broke the concept of GRID. (This makes a big difference when you have many clients i.e.: Over 20 clients.)
              If we remove or add node we must configure the clients to use/or not this node. Using SCAN he do this automatically.

              How the things works?
              You configure only one host on your client (e.g host-scan). When the client requests a connection, Oracle Client 11gR2 searching for IPs available for this host-scan. So, he try connect on cluster using one of IPs avaliable.
              The SCAN Listener will receive this connection and re-direct to the node (Host Listener) with less workload from this point the the connection is established using Virtua IP (VIP).
              I can say : SCAN is the connection manager with high availability.

              Why 3 SCAN IP?
              Sebastian answered this very well.
              Re: Are 3 IPs required for a SCAN in 11.2?

              This is the purpose of SCAN IP. I hope you understand.

              Regards,
              Levi Pereira

              Edited by: Levi Pereira on Apr 29, 2011 10:10 PM
              • 4. Re: Are 3 IPs required for a SCAN in 11.2?
                dba69
                Thanks again. I understand the concept of SCAN. For the sake of argument and time, what if I have a strong business reason for not using it, namely, not enough IPs to use without doing a major reworking of our network.

                What is the BARE MINIMUM that needs to be configured in regard to SCAN to keep the cluster up and working as it is now? We don't need scalability as we won't be adding more nodes to this cluster.

                All the client accessing the cluster right now are working fine connecting through a service we created in the cluster. Will that continue to work as it has been?
                • 5. Re: Are 3 IPs required for a SCAN in 11.2?
                  Levi Pereira
                  Hi,
                  dba69 wrote:
                  What is the BARE MINIMUM that needs to be configured in regard to SCAN to keep the cluster up and working as it is now? We don't need scalability as we won't be adding more nodes to this cluster.
                  You must set at least one IP to SCAN.
                  All the client accessing the cluster right now are working fine connecting through a service we created in the cluster. Will that continue to work as it has been?
                  Yes...You can keep using VIP or Public (not recommended) or use SCAN with one SCANIP. The three options are valid, just make sure that depending on the method chosen will impact in their business.

                  Levi Pereira