On Wed, May 26, 2010 at 2:54 PM, Ran Tavory <firstname.lastname@example.org
> If I disable row cache the numbers look good - key cache hit rate is > 0, so
> it seems to be related to row cache.
> Interestingly, after running for a really long time and with both row and
> keys caches I do start to see Key cache hit rate > 0 but the numbers are so
> small that it doesn't make sense.
> I have capacity for 10M keys and 10M rows, the number of cached keys is ~5M
> and very similarly the number of cached rows is also ~5M, however the hit
> rates are very different, 0.7 for rows and 0.006 for keys. I'd expect the
> keys hit rate to be identical since none of them reached the limit yet.
> Key cache capacity: 10000000
> Key cache size: 5044097
> Key cache hit rate: 0.0062089764058896576
> Row cache capacity: 10000000
> Row cache size: 5057231
> Row cache hit rate: 0.7361241352465543
> On Tue, May 25, 2010 at 3:43 PM, Jonathan Ellis <email@example.com
>> What happens if you disable row cache?
>> On Tue, May 25, 2010 at 4:53 AM, Ran Tavory <firstname.lastname@example.org
>> > 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?
>> Jonathan Ellis
>> Project Chair, Apache Cassandra
>> co-founder of Riptano, the source for professional Cassandra support