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Overview of traditional snapshots

Software and hardware snapshots are also called traditional snapshots.

A traditional snapshot allows the supported utility to process data while a database remains available for updates. When the snapshot process starts, the database takes a very brief outage to establish a point of consistency.

At this point, XBM starts to provide the data to the supported utility consistent with that point of time:

  • For software snapshots, XBM monitors write requests to the database for the data objects being processed. When a record is changed, XBM stores a preimage of the record in its software cache.
  • For hardware snapshots, XBM uses intelligent storage to provide preimage records from a 'frozen' copy of the database to the utility.

As the utility reads database records during its job, XBM satisfies the read request of the utility with the preimage from either the hardware device or software cache. In this manner the data read by the utility for that database is as it existed when the point of consistency was established, while the source database continues to be updated.

The following sections describe these processes in more detail.


 

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SNAPSHOT UPGRADE FEATURE for VSAM 6.1