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From Ran Tavory <ran...@gmail.com>
Subject Error reporting Key cache hit rate with cfstats or with JMX
Date Tue, 25 May 2010 09:53:36 GMT
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?

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