IBM TotalStorage 3494 Tape Library, Operator'S Manual

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Emulation of 3490E Tape Drives
From a host perspective, the VTS subsystem looks like two, four, eight, or sixteen
3490E control units, each with 16 tape drives. Each emulated drive is called a
virtual tape drive. The subsystem handles all 3494 Tape Library tape commands.
Emulating a 3490E-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- and 3592-type
tape drives. There is no direct relationship between a virtual tape drive and a real
3590 or 3592 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
first 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.
For the 3494 B18 VTS, the amount of data that is 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 400 MB or 800 MB of data,
respectively, without data compression. With data compression provided by the
ESCON High Performance Option feature or the Extended High Performance
Option feature, the actual host data stored on a virtual Cartridge System Tape or
Enhanced Capacity Cartridge System Tape volume can be up to 1.2 GB or 2.4 GB,
respectively (assuming a 3:1 compression).
For the 3494 B10 and B20 VTSs, the amount of data stored on a virtual volume is
selectable based on the data class The size defined by the data class can be:
v
Insert media type (sets the logical volume size to that of the media type that was
selected when the logical volumes were inserted; either 400 MB or 800 MB)
v 1000 MB (1 GB)
v
2000 MB (2 GB)
v 4000 MB (4 GB; only available if the 3494 B10 or B20 VTS has 1.7TB cache and
3592 J1A Tape Drives or 3592 E05 Tape Drives emulating J1A attached)
See “Manage Data Classes” on page 264 for information on setting virtual volume
capacity.
All host interactions with data in a VTS are through virtual volumes and
associated virtual tape drives; there is no direct access to the data on a physical
cartridge or drive.
Tape Volume Cache
The size of the disk storage is large enough so more virtual volumes can be
retained in it than just the ones currently associated with the virtual drives. After
an application closes a virtual volume, if it was modified, a copy of it is made by
the storage management software onto a physical tape. The virtual volume remains
available on the disk storage until the space it occupies is needed to satisfy another
mount request. Leaving the virtual volume in the disk storage allows for fast
access to it during a subsequent request for the volume. The disk storage, and
management of that space to keep volumes available after they are closed, is called
the tape volume cache . The performance for mounting of a volume that is in the tape
volume cache is quicker than if a real physical volume is mounted. Disk storage, in
effect, caches the tape volumes and provides for fast access.
Chapter 3. Operational Characteristics
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