How caching works 

Brief overview of how caching works.

When both row cache and partition key cache are configured, the row cache returns results whenever possible. In the event of a row cache miss, the partition key cache might still provide a hit that makes the disk seek much more efficient. This diagram depicts two read operations on a table with both caches already populated.

One read operation hits the row cache, returning the requested row without a disk seek. The other read operation requests a row that is not present in the row cache but is present in the partition key cache. After accessing the row in the SSTable, the system returns the data and populates the row cache with this read operation.