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Thread: RAC Concepts Q & A

  1. #1
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    RAC Concepts Q & A

    What is Cache Fusion and how does this affect applications?

    Cache Fusion is a shared cache architecture that uses high speed low latency interconnects available today on clustered systems to maintain database cache coherency. Database blocks are shipped across the interconnect to the node where access to the data is needed. This is accomplished transparently to the application and users of the system. As Cache Fusion uses at most a 3 point protocol, this means that it easily scales to clusters with a large numbers of nodes. For more information about cache fusion see the following link:

    Additional Information can be found at:

    Cache-Fusion

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    How to transition from Single Instance to RAC?

    If the cluster and the cluster software are not present, these components must be installed and configured. The RAC option must be added using the Oracle Universal Installer, which necessitates the existing DB instance must be shut down. There are no changes necessary on the user data within the database. However, a shortage of freelists and freelist groups can cause contention with header blocks of tables and indexes as multiple instances vie for the same block. This may cause a performance problem and require data partitioning. However, the need for these changes should be rare.

    Recommendation: apply automatic space segment management to perform these changes automatically. The free space management will replace the freelists and freelist groups and is better. The database requires one Redo thread and one Undo tablespace for each instance, which are easily added with SQL commands or with Enterprise Manager tools.

    Datafiles will need to be moved to either a clustered file system (CFS) or raw devices so that all nodes can access it. Also, the MAXINSTANCES parameter in the control file must be greater than or equal to number of instances you will start in the cluster.

    For more detailed information, please see

    Convert-Single-Instance-Database-to-RAC


    With Oracle Database 10g Release 2, $ORACLE_HOME/bin/rconfig tool can be used to convert Single instance database to RAC. This tool takes in a xml input file and convert the Single Instance database whose information is provided in the xml. You can run this tool in "verify only" mode prior to performing actual conversion. This is documented in the RAC admin book and a sample xml can be found $ORACLE_HOME/assistants/rconfig/sampleXMLs/ConvertToRAC.xml. This tool only supports databases using a clustered file system or ASM. You cannot use it with raw devices. Grid Control 10g Release 2 provides a easy to use wizard to perform this function.

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    WARNING: No cluster interconnect has been specified. I get this error starting my RAC database, what do I do?

    It simply means you do not have cluster_interconnects parameter set and nothing was set in the OCR, so the private interconnect is picked at random by the database and hence the warning...
    You can either set cluster_interconnects parameter in the init.ora to the private interconnect IP; OR play with oifcfg getif and setif (type oifcfg without anything for help message)

    $ oifcfg getif
    eth0 138.2.236.0 global public
    eth2 138.2.238.0 global cluster_interconnect

    What does your output look like?

    Note that if hardware is not identical you'll have to provide each node with it's own correct value, if it's identical hardware you can use the -global switch.

    What kind of HW components do you recommend for the interconnect?

    The general recommendation for the interconnect is to provide the highest bandwith interconnect, together with the lowest latency protocol that is available for a given platform. In practice, Gigabit Ethernet with UDP has proven sufficient in every case it has been implemented, and tends to be the lowest common denominator across platforms.

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    Why is validateUserEquiv failing during install (or cluvfy run)?

    SSH must be set up as per the pre-installation tasks. It is also necessary to have file permissions set as described below for features such as Public Key Authorization to work. If your permissions are not correct, public key authentication will fail, and will fallback to password authentication with no helpful message as to why.
    SSH (from OUI) will also fail if you have not connected to each machine in your cluster as per the note in the installation guide:

    The first time you use SSH to connect to a node from a particular system, you may see a message similar to the following:

    The authenticity of host 'rac1 (...)' can't be established. RSA key fingerprint is ...
    Are you sure you want to continue connecting (yes/no)?

    Enter |yes| at the prompt to continue. You should not see this message again when you connect from this system to that node. Answering yes to this question causes an entry to be added to a "known-hosts" file in the .ssh directory which is why subsequent connection requests do not re-ask.

    The user (oracle) should be able to ssh from one node to another , without requirement of any password prior to installation.

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    How does OCR mirror work? What happens if my OCR is lost/corrupt?

    OCR is the Oracle Cluster Registry, it holds all the cluster related information such as instances, services. The OCR file format is binary and starting with 10.2 it is possible to mirror it. Location of file(s) is located in: /etc/oracle/ocr.loc in ocrconfig_loc and ocrmirrorconfig_loc variables.

    Obviously if you only have one copy of the OCR and it is lost or corrupt then you must restore a recent backup, see ocrconfig utility for details, specifically -showbackup and -restore flags. Until a valid backup is restored the Oracle Clusterware will not startup due to the corrupt/missing OCR file.

    The interesting discussion is what happens if you have the OCR mirrored and one of the copies gets corrupt? You would expect that everything will continue to work seemlessly. Well.. Almost.. The real answer depends on when the corruption takes place.

    If the corruption happens while the Oracle Clusterware stack is up and running, then the corruption will be tolerated and the Oracle Clusterware will continue to funtion without interruptions. Despite the corrupt copy. DBA is advised to repair this hardware/software problem that prevent OCR from accessing the device as soon as possible; alternatively, DBA can replace the failed device with another healthy device using the ocrconfig utility with -replace flag.


    If however the corruption happens while the Oracle Clusterware stack is down, then it will not be possible to start it up until the failed device becomes online again or some administrative action using ocrconfig utility.

    When the Clusteware attempts to start and fails , we should check the system logs (/var/log/messages) and the logs in the $ORA_CRS_HOME/logs/<node name> , to get the error details , and Fix whatever problem (hardware/software?) .

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    Why do we have a Virtual IP (VIP) in Oracle RAC 10g? Why does it just return a dead connection when its primary node fails?

    The goal is application availability.

    When a node fails, the VIP associated with it is automatically failed over to some other node. When this occurs, the following things happen.
    (1) VIP detects public network failure which generates a FAN event.
    (2) the new node re-arps the world indicating a new MAC address for the IP.
    (3) connected clients subscribing to FAN immediately receive ORA-3113 error or equivalent. Those not subscribing to FAN will eventually time out.
    (4) New connection requests rapidly traverse the tnsnames.ora address list skipping over the dead nodes, instead of having to wait on TCP-IP timeouts
    Without using VIPs or FAN, clients connected to a node that died will often wait for a TCP timeout period (which can be up to 10 min) before getting an error.
    As a result, you don't really have a good HA solution without using VIPs and FAN. The easiest way to use FAN is to use an integrated client with Fast Connection Failover (FCF) such as JDBC, OCI, or ODP.NET.


    What does the Virtual IP service do?

    The 10g Virtual IP Address (VIP) exists on every RAC node for public network communication. All client communication should use the VIPs in their TNS connection descriptions. The TNS ADDRESS_LIST entry should direct clienst to VIPs rather than using hostnames. During normal runtime, the behaviour is the same as hostnames, however when the node goes down or is shutdown the VIP is hosted elsewhere on the cluster, and does not accept connection requests. This results in a silent TCP/IP error and the client fails immediately to the next TNS address. If the network interface fails within the node, the VIP can be configured to use alternate interfaces in the same node. The VIP must use the public interface cards.

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    What do the VIP resources do once they detect a node has failed/gone down? Are the VIPs automatically acquired, and published, or is manual intervention required? Are VIPs mandatory?


    When a node fails, the VIP associated with the failed node is automatically failed over to one of the other nodes in the cluster. When this occurs, two things happen:
    The new node re-arps the world indicating a new MAC address for this IP address. For directly connected clients, this usually causes them to see errors on their connections to the old address;

    Subsequent packets sent to the VIP go to the new node, which will send error RST packets back to the clients. This results in the clients getting errors immediately.
    In the case of existing SQL conenctions, errors will typically be in the form of ORA-3113 errors, while a new connection using an address list will select the next entry in the list. Without using VIPs, clients connected to a node that died will often wait for a TCP/IP timeout period before getting an error. This can be as long as 10 minutes or more. As a result, you don't really have a good HA solution without using VIPs.

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    How do we apply a Patch to RAC Instances

    Please refer to :
    Strategies-to-Apply-Patch-to-RAC-Instances&p=567#post567

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    What is the Load Balancing Advisory?

    To assist in the balancing of application workload across designated resources, Oracle Database 10g Release 2 provides the Load Balancing Advisory. This Advisory monitors the current workload activity across the cluster and for each instance where a service is active; it provides a percentage value of how much of the total workload should be sent to this instance as well as service quality flag. The feedback is provided as an entry in the Automatic Workload Repository and a FAN event is published. The easiest way for an application to take advantage of the load balancing advisory, is to enable Runtime Connection Load Balancing with an integrated client.


    What is Runtime Connection Load Balancing?

    Runtime connection load balancing enables the connection pool to route incoming work requests to the available database connection that will provide it with the best service. This will provide the best service times globally, and routing responds fast to changing conditions in the system. Oracle has implemented runtime connection load balancing with ODP.NET and JDBC connection pools. Runtime Connection Load Balancing is tightly integrated with the automatic workload balancing features introduced with Oracle Database 10g I.E. Services, Automatic Workload Repository, and the new Load Balancing Advisory.


    How do I enable the load balancing advisory?

    The load balancing advisory requires the use of services and Oracle Net connection load balancing.
    To enable it, on the server: set a goal (service_time or throughput, and set CLB_GOAL=SHORT ) on your service.
    For client, you must be using the connection pool.
    For JDBC, enable the datasource parameter FastConnectionFailoverEnabled.
    For ODP.NET enable the datasource parameter Load Balancing=true.

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    What is the Cluster Verification Utiltiy (cluvfy)?

    The Cluster Verification Utility (CVU) is a validation tool that you can use to check all the important components that need to be verified at different stages of deployment in a RAC environment. The wide domain of deployment of CVU ranges from initial hardware setup through fully operational cluster for RAC deployment and covers all the intermediate stages of installation and configuration of various components. Cluvfy does not take any corrective action following the failure of a verification task, does not enter into areas of performance tuning or monitoring, does not perform any cluster or RAC operation, and does not attempt to verify the internals of cluster database or cluster elements.

    For Detail Usage, Refer to : Cluvfy-the-cluster-verification-tool

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