Span and Snapshot Compared

The Span and Snapshot accessories both extract the DMSII database to flat sequential disk files or tape. The following table includes a comparison of these accessories to help you choose the most suitable replication method.

Span

Snapshot

Most useful when you have frequent, ongoing database changes that you need to track.

Most useful for one-time clones. Also useful for gathering periodic information (such as month-end information that can be queried from a database).

Cleanup and consolidation occur on the secondary database system (requires fewer mainframe resources than Snapshot).

Cleanup and consolidation occur on the mainframe.

Maintains audit location information about each replicated data set. This information is used to determine where to start retrieving changes that it finds in the audit trail. This quiet point location is stored in the DATA/SPAN/databasename/CONTROL file on the host.

Records in the data files created by the Snapshot Accessory represent the same QPT (quiet point) in the audit trail.

Provides updates to the secondary database (only database changes are gathered).

When the Span Accessory requests updates for data sets that have not been cloned, DBEngine initiates a routine to extract all of the records for the uncloned data sets and returns these records to the Span Accessory as newly-created records. In addition, DBEngine forwards any changes that occurred to these records during the extraction.

To update the database, you must run the Snapshot Accessory again to reclone it.

Span's update data must be loaded to the secondary database (for example, via a user-written program).

Snapshot's output data files must be transported to the secondary database.