Virtual device operation

Virtualization enables the TS7700 to appear as a collection of tape drives to an accessing host.

The virtual tape drives of a TS7700 are defined by using the same characteristics of physical tape drives. As a result, from the perspective of an accessing host the TS7700 appears as a collection of tape drives. Depending on the machine model and installed features, this collection provides up to 31 logical control units and up to 496 virtual drives. Virtual drives are organized in groups of 16 drive addresses under a single logical control unit address. Like a physical tape drive, each virtual drive has the following characteristics:
  • Has a host device address
  • Is included in the I/O generation for the system
  • Can be varied online or offline to the host
  • Supports all dynamic path creation and dynamic partitioning modes
  • Signals ready when a virtual volume is loaded
  • Supports all IBM® 3490 tape I/O commands
  • Supports host data compression controls
  • Exits the ready state when a virtual volume is rewound and unloaded
  • Maintains statistical counters that can be retrieved as buffered log data
A virtual tape drive differs from a physical tape drive in two important ways. First, the "media" for a virtual tape drive is a direct access storage device (DASD), which is suited for the stop-start transfer of data with low performance impact. By contrast, the media for a physical tape drive is a physical tape. When this physical tape is in motion, the host must continue to write data at a rate no slower than the rate at which data is written to the media, or the tape must stop. If the tape stops, it is repositioned, and assuming that sufficient data is buffered to write, started again. This stop-start motion is time consuming and contributes to the wear of tape media and tape drives. For 3490 and later class drives, few applications can sustain the bandwidth that is needed to 'stream' the drives to prevent frequent repositioning.

Second, the resources in a virtual drive are embodied in software and electronic storage (buffering). If the host is not sending data to a virtual drive, the drive does not consume processor cycles or storage space. Those resources remain available for use by other virtual drives. By comparison, a physical tape drive resource is dedicated to the host application that is using it for while the volume is mounted. If the host is not sending data to the drive at a high rate because of the nature of the application, the resource is underutilized.