If the user specifies both volume and unit information during allocation of a non-SMS-managed volume or if the allocation process described in Recall volume selection is not done for the data set, a migrated data set to be opened will still be on a migration volume when the open routine tries to open the VSAM data set. The VSAM open routine issues a catalog locate request to get information about the cluster from the catalog. DFSMShsm intercepts the locate request, recalls the data set to the primary volume it migrated from, and returns the catalog information that the VSAM open routine requested. Enough space must exist for DFSMShsm to recall the data set to the volume the open routine specifies; otherwise, the VSAM open routine fails.