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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
significant 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:
v Emulation of 32 or 64 3490-type tape drives
v Tape volume cache
v Storage management of the tape volume cache
v Maintaining data fragments from copied volumes
v Fast response for non-specific 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
© Copyright IBM Corp. 1993, 1999 41