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Write caching

If the drive has a nonvolatile cache, such as battery-backed RAM, then as many write requests as possible are accepted and reported as written, then the drive takes its time to write the data onto the actual physical disk. Otherwise data is not reported as written until it is actually written. If there is a write hit, then the copy in the cache is written out before the new copy is accepted into the cache. [RW94]

Barnett Hsu