Configuring data caches
Cassandra includes integrated caching and distributes cache data around the cluster for you. When a node goes down, the client can read from another cached replica of the data. The integrated architecture also facilitates troubleshooting because there is no separate caching tier, and cached data matches what’s in the database exactly. The integrated cache solves the cold start problem by virtue of saving your cache to disk periodically and being able to read contents back in when it restarts—you never have to start with a cold cache.
About the partition key cache
The partition key cache is a cache of the partition index for a Cassandra table. Using the key cache instead of relying on the OS page cache saves CPU time and memory. However, enabling just the key cache results in disk (or OS page cache) activity to actually read the requested data rows.
About the row cache
The row cache is similar to a traditional cache like memcached: when a row is accessed, the entire row is pulled into memory (merging from multiple SSTables if necessary) and cached so that further reads against that row can be satisfied without hitting disk at all.
Typically, you enable either the partition key or row cache for a table. The main exception is for archive tables that are infrequently read. You should disable caching entirely for archive tables.