IBM Magstar 3494 manual Storage Management of the Tape Volume Cache

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 modi®ed, 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 ®les 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 ®rst several records from the last use of the volume. Normally, the ®rst 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-speci®c Mount Requests

When a non-speci®c 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-speci®c mount in a 3494 by specifying a category instead of a speci®c volser in the mount request. The library manager then selects the next available volume assigned to the speci®ed 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-speci®c mounts. Categories used for non-speci®c mounts are de®ned through the library manager as ªFast Readyº categories (refer to Figure 98 on page 167). When a mount request speci®es a category de®ned as ªFast Readyº, the mount is satis®ed by accessing the data fragment in the tape volume cache associated with

42Magstar 3494 Tape Library Operator Guide