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Many higher-end SCSI controllers have built-in caches. The idea behind a cache is to use high-speed memory to hold recently retrieved results, to save time if the results are needed again in the near future. This improves performance because most SCSI devices are drives, much slower than memory in relative terms. Caching is a concept that is used extensively within the PC world; it is found in CPUs, hard disks, optical drives and a variety of other devices. To read more about caching in general terms, see this section.
Caching that is done by the SCSI controller adds an additional caching level that exists, logically, in front of the cache (buffer) that resides within SCSI hard disks or other components. When data is requested from a device on the SCSI bus, the host adapter sees if it is already in its internal cache and if so, returns the results much more quickly, saving transaction bandwidth on the SCSI bus at the same time. This of course improves performance over a system that does not have this type of cache.
The amount of cache memory on a host adapter depends on the model; some are user-upgradeable. Note that there are diminishing returns associated with cache memory; each megabyte you add helps performance less than the previous megabyte did...