IBM Magstar 3494 manual 3490E/3590 Tape Subsystem Operation, Virtual Tape Server

Chapter 3. Operational Characteristics

The operational characteristics of the 3494 are described in this chapter.

3490E/3590 Tape Subsystem Operation

The 3494 controls the loading, unloading, and affiliated operation of the tape subsystem in the library. No operator attendance is required unless the library is unable to recover from a subsystem problem.

In addition to existing tape subsystem error recovery, the control unit and the library manager execute additional recovery procedures when the tape drive detects a load, unload, or tension loss failure.

Virtual Tape Server

The IBM Magstar 3494 Model B16 or B18 Virtual Tape Server provides for a higher utilization of 3590 tape technology than enabled by current tape controller concepts. A Virtual Tape Server provides the improvement in utilization without impacting current operating system or independent software vendors. The subsystem combines the random access and high performance characteristics of disk storage with outboard hierarchical storage management and virtual tape drives to provide signi®cant reductions in the number of physical cartridges, tape drives, and automated libraries needed to store the customer tape data. The key concepts for the architecture of the subsystem are:

vEmulation of 32 or 64 3490-type tape drives

vTape volume cache

vStorage management of the tape volume cache

vMaintaining data fragments from copied volumes

vFast response for non-speci®c mount requests

Emulation of 3490-Type Tape Drives

From a host perspective, the Virtual Tape Server subsystem looks like two or four 3490E control units, each with 16 tape drives. Each emulated drive is called a virtual tape drive. The subsystem handles all 3490 tape commands. Emulating a 3490±type tape drive eliminates the need for host software support of a new type of tape drive in order to utilize the capacity of 3590±type tape drives. There is no direct relationship between a virtual tape drive and a real 3590 tape drive.

Data is written and read as if it is stored on a real Cartridge System Tape or an Enhanced Capacity Cartridge System Tape. However, within the subsystem, data is stored on disks. All tape read and write commands are translated to read and write data records from or to disk storage. Tape marks are stored as special records on the disk storage as well. Volumes residing on disk storage are called virtual volumes.

The amount of data stored on a virtual volume is variable up to a maximum as determined by the media type selected. Two media types are emulated (standard Cartridge System Tape and Enhanced Capacity Cartridge System Tape); they can hold up to 400MB or 800MB of customer data, respectively. The 3494 Model B18

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