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Thread: Instance Recovery in RAC

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    Instance Recovery in RAC

    The CRS Daemons first detects the node and instance failure. It communicates the failure status to the GCS by way of the LMON process. At this stage, any surviving instance in the cluster initiates the recovery process.

    1) During the first phase of recovery, which is the GES reconfiguration, Oracle first reconfigures the GES enqueues. Then Oracle reconfigures the GCS resources. During this time, all GCS resource requests and write requests are temporarily suspended. However, processes and transactions can continue to modify data blocks as long as these processes and transactions have already acquired the necessary enqueues.

    2) After the reconfiguration of enqueues that the GES controlled, a log read and the remastering of GCS resources occur in parallel.LMON is doing the remastering and SMON is determining the recovery set using the edo logs of the failed instances. At the end of this step the block resources that need to be recovered have been identified.

    If multiple instance have failed then a redo merge of the redo logs of all the failed instances would be used to determine the recovery set

    3) Buffer space for recovery is allocated and the resources that were idenetified in the previous reading of the log are claimed as recovery resources. (Therefore size the Buffer Cache for RAC at least 25% more than for Single Instances)
    Then, assuming that there are PIs of blocks to be recovered in other caches in the cluster database, resource buffers are requested from other instances. The resource buffers are the starting point of recovery for a particular block.

    4) All resources and enqueues required for subsequent processing have been acquired and the Global Resource Directory is now unfrozen. Any data blocks that are not in recovery set can now be accessed. Note that the system is already partially available.

    5) The cache layer recovers and writes each block identified in step 2, releasing the recovery resources immediately after block recovery so that more blocks become available as cache recovery proceeds.

    6) After all blocks have been recovered and the recovery resources have been released, the system is again fully available. Recovered blocks are available after recovery completes.

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