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.
~ Hardware disk caches and system memory caches for stand alone systems.
~ Disk storage caches for optical disk libraries.
~ 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.
Cache Organization for Hierarchical Storage Systems: The hierarchical, storage management system consists of at least three or four types of storage as follows:
· System memory Cache
· On-line high speed magnetic disk storage
· Near-line optical disk libraries
· Off-line optical tape storage
Many Cache designs use a high-water mark and a low-water mark to trigger cache management operations. When the cache storage fills up to the high-water mark, the cache manager starts creating more space in cache storage. Space is created by discarding objects.
o The cache manager maintains a data base of objects in the cache. Cache areas containing updated objects are frequently called dirty cache.
o Objects in dirty cache are written back at predetermined time intervals or before discarding an object.