Apache Cassandra 0.7 Documentation  Maximizing Cache Benefit
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Cassandra’s built-in key and row caches can provide very
efficient data caching. Some Cassandra production deployments
have leveraged Cassandra’s caching features to the point where
dedicated caching tools such as memcached could be completely
replaced. Such deployments not only remove a redundant layer
from the stack, but they also achieve the fundamental efficiency
of strengthening caching functionality in the lower tier where
the data is already being stored. Among other advantages, this
means that caching never needs to be restarted in a completely
“cold” state.  To fully optimize Cassandra’s caching hit rate,
several aspects of the system should be considered, from
hardware basics to specific caching thresholds. With proper
tuning, hit rates of 85% or better are possible with Cassandra,
and each hit on a key cache can save one disk seek per SSTable.
Row caching, when feasible,