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Hierarchical Storage Applications
Banks, insurance companies, hospitals, state and federal governments,
manufacturing companies and a variety of other business and service
organizations need to permanently store large volumes of their records, from
simple documents to video information, for audit trail use.

Cache Management for Storage Systems

Disk caches are an integral part of a hierarchical storage management
architecture. Hierarchical storage consists of a number of media classes, ranging
from high speed and expensive on-line fast cache storage to low-cost off-line
storage.

Role of on-line caches: The primary role of on-line caches as used in document-
imaging systems is to provide high speed on-line storage for documents currently
in use that may be accessed in the future. This role can be extended to multimedia
systems. Hierarchical Organization of Caches: Caches are used at various storage
levels.

The following lists representative storage systems using cache storage.

o Hardware disk caches and system memory caches for standalone systems.
o Disk storage caches for optical disk libraries.
o Disk storage caches for networked systems.

Low-level Disk Caching Disk

Caching Controllers: Two approaches to implement disk caching controllers are:
in hardware and software. A hardware caching controller is designed with its own
on-board CPU and private memory.

The private memory is used for storing disk data temporarily. It is known as disk
cache. When an 1/0 request is received by the caching controller, the CPU on the
caching controller initiates sectors of data, including sectors that contain the
requested data. Disk writes managed through a disk cache can be delayed writes
or write-throughs. For delayed through, data is written by the host CPU to the
disk cache, and the caching controller writes the data from the disk cache to the
disk when read activity is low.