The EMC CLARiiON uses cache in a manner similar to how cache memory is used in traditional workstations and servers. With servers and workstations, application code and application data locality and immediacy are the focus of the design. A characteristic of many applications (including file systems) is to cache new data locally, then periodically flush the data to the actual storage device. This “lazy write” approach would result in large bursts of large I/Os to the storage systems - a perfect fit with the “burst smoothing” benefit of CLARiiON’s caching. Cache page size is perhaps the most influential parameter on cache performance. The division between staging and storage memory is not formally defined by addresses, but more by functionality - as data arrives, it is always staged, and as needed, it may be marked for storage as well. The Partition Memory dialog in Navisphere Manager enables part of the cache to be used for storage operations. Unlike the Symmetrix, certain writes may bypass cache memory.
Salient Features :-
1) Cache memory on an SP performs two tasks.
Staging: Temporary buffering of current read and write data.
Always performed on each I/O.
Storage: Repository for frequently accessed data.
Maintaining copies of read and write data.
User must explicitly enable this (for both read and write).
2) Benefits of caching
Burst Smoothing - Absorb bursts of writes without becoming “disk bound”.
Write cache optimization.
Locality - Merge several writes to the same area into a single operation.
Increases write performance.
Immediacy - Satisfy user requests without going to the disks.
Read cache optimization prefetching of data for sequential reads.
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