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How XBM and snapshot utilities populate the extended buffer

When a configuration and a management set containing a snapshot template are activated and a snapshot utility job is then started, the utility registers the objects that will be processed with XBM.

The following steps describe this process:

  1. XBM targets the registered objects as active.
  2. The utility establishes a point of consistency (a quiesce point). How the quiesce point is established depends on the utility being used.
  3. The snapshot job starts. As the snapshot job progresses, XBM monitors write requests that update the targeted objects.

  4. If a write request is issued for a targeted object, XBM places an image of the page before it is updated (called a preimage ) in the extended buffer. If the utility subsequently issues a read request for the updated page, XBM satisfies the request by using the preimage from the extended buffer.

    If the preimage is needed by the utility but is not available in cache (because of a cache failure or some other problem), XBM sends a return code and reason code to the utility. The utility determines whether to continue or terminate the operation.

  5. After the utility uses the preimage, the used preimage is removed from the XBM cache.

Note

For restartable snapshot jobs, preimages continue to be cached in the extended buffer, though the utility job might have temporarily stopped. XBM does not remove the preimages from the extended buffer until the job is complete or you manually terminate the job (for example, by specifying TERM SNAPSHOT). For information about restartable parameter keywords, options, and use, see the documentation for the supported utility.

Related topic

Overview-of-the-extended-buffer


 

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