10. Storage Hierarchy

• Storage systems organized in hierarchy.
– Speed
– Cost
– Size
– Volatility

STORAGE HEIRARCHY(MEMORY HEIRARCHY)


-> CACHING
copying information into faster storage system; main memory can be viewed as a cache for secondary storage.

• improve performance where a large access-time or transfer-rate disparity exists between two components) Memory caching: add cache (faster and smaller memory) between CPU and main memory
--> When need some data, check if it’s in cache
-->If yes, use the data from cache
--> If not, use data from main memory and put a copy in cache
– Disk caching: main memory can be viewed as a cache for disks

-> Cache COHERENCY and CONSISTENCY
• Cache coherency in multiprocessor systems
- Each CPU has a local cache
- A copy of X may exist in several caches --> must make sure that an update of X in one cache is immediately reflected in all other caches where X resides
- Hardware problem

• Cache consistency in distributed systems
- A master copy of the file resides at the server machine
- Copies of the same file scattered in caches of different client machines
- Must keep the cached copies consistent with the master file
- OS problem