The TS7720 Virtualization Engine, displayed by the host as a 3957 Model VEx, is similar in concept to the TS7740; however, what makes it different is that it provides support for a disk-only solution. The TS7720 is a virtual tape solution that has no back-end physical tape. Because all of the data written to the TS7720 Virtualization Engine resides in cache, the TS7720 supports larger amounts of cache than the TS7740. The TS7720 is also supported in a multi-grid configuration and starting with Release 1.6 of the TS7700, the clusters in a grid can be heterogeneous (TS7720 and TS7740), referred to as a hybrid configuration. Like a TS7740, the TS7720 is also defined to the host using composite and distributed libraries.

When the TS7720 detects that available cache space has fallen below preset thresholds, the TS7720 sends CBR3792E (limited cache free space warning state reached) and CBR3794A (out of cache resource critical state reached) attention messages to the attached hosts. The CBR3792E messages can be used to trigger return to scratch processing or the copying of data to another library. if the amount of available cache subsequently reaches the critical state, all fast ready (scratch) mounts are failed and any specific mount operations are allowed; however, any attempt to write to the volume will be failed. Mount operations that have been accepted before entering this state complete and volumes currently mounted can continue to perform host I/O operations. As appropriate, the VARY SMS,STORGRP operator command can also be used to steer scratch allocations to another composite library that is eligible for the scratch request. The DISPLAY SMS,LIBRARY command with DETAIL, can also be used periodically to display the "CACHE PERCENTAGE USED" in a distributed library.

To assist with the cache management in a TS7720, having a TS7740 Virtual Engine in the multi-cluster grid configuration (hybrid configuration) provides an additional back-end tape capability. To manage the migration of data from the TS7720 to the TS7740, with Release 1.6 of the TS7700, there is a ‘Volume Removal Policy’ at the library. Since the TS7720 has a finite amount of cache, once that cache fills, this policy allows logical volumes to be automatically removed from cache while a copy is retained elsewhere in the grid. This copy could exist on physical tape associated with a TS7740 or in another TS7720 tape volumes cache. When coupled with copy policies, this provides an automatic data migration function between the clusters in the grid.