IBM Tape Library Magstar 3494, Руководство оператора

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with the ESCON High Performance Option (EHPO) feature emulates storage of up
to 2.4 GB (3:1 compression) on the virtual Enhanced Capacity Cartridge System
Tape.
All host interactions with data in a Virtual Tape Server subsystem 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 in the subsystem 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.
Storage Management of the Tape Volume Cache
Storage management software in the subsystem manages the contents of the tape
volume cache. Virtual tape volumes are copied from the tape volume cache to
physical tape when the virtual volume has been closed, and they are recalled from
tape to the tape volume cache when they are again requested to be mounted. The
storage management software stacks multiple migrated files onto a 3590 tape,
thereby utilizing its storage capacity.
Maintaining Data Fragments from Copied Volumes
When the cache space occupied by a closed virtual tape volume is needed for
other active virtual volumes, the data it represents is not completely removed. A
fragment of the data is kept on disk storage. The data fragment includes information
about the copied virtual volume so that it can be recalled and it also includes the
first several records from the last use of the volume. Normally, the first few records
on a tape contain a tape volume label, and enough data records are maintained to
contain an IBM standard tape label plus any unique user label records.
Fast Response for Non-specific Mount Requests
When a non-specific mount is requested, the customer application is going to write
data from the beginning of tape, overwriting any existing data on the tape. The host
can request a non-specific mount in a 3494 by specifying a category instead of a
specific volser in the mount request. The library manager then selects the next
available volume assigned to the specified category to satisfy the host request.
Within a Virtual Tape Server subsystem, the data fragment is used in conjunction
with a mount from category request to provide very fast response times for
non-specific mounts. Categories used for non-specific mounts are defined through
the library manager as “Fast Ready” categories (refer to Figure 98 on page 167).
When a mount request specifies a category defined as “Fast Ready”, the mount is
satisfied by accessing the data fragment in the tape volume cache associated with
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Magstar 3494 Tape Library Operator Guide