It seems there's an error reporting the Key cache hit rate. The value is always 0.0 and I have a feeling it's incorrect. This is seen both by using notetool cfstats as well as accessing JMX directly (org.apache.cassandra.db:type=Caches,keyspace=outbrain_kvdb,cache=KvAdsKeyCache RecentHitRate)
<ColumnFamily CompareWith="BytesType" Name="KvAds" RowsCached="10000000" KeysCached="10000000"/> Column Family: KvAds SSTable count: 7 Space used (live): 1288942061 Space used (total): 1559831566 Memtable Columns Count: 73698 Memtable Data Size: 17121092 Memtable Switch Count: 33 Read Count: 3614433 Read Latency: 0.068 ms. Write Count: 3503269 Write Latency: 0.024 ms. Pending Tasks: 0 Key cache capacity: 10000000 Key cache size: 619624 Key cache hit rate: 0.0 Row cache capacity: 10000000 Row cache size: 447154 Row cache hit rate: 0.8460295730014572 Compacted row minimum size: 387 Compacted row maximum size: 31430 Compacted row mean size: 631 The Row cache hit rate looks good, 0.8 but Key cache hit rate always seems to be 0.0 while the number of unique keys stays about 619624 for quite a while. Is it a real caching problem or just a reporting glitch?