It sure sounds like you're seeing the "my row cache contains the entire hot data set, so the key cache only gets the cold reads" effect.
On Wed, May 26, 2010 at 2:54 PM, Ran Tavory <ran...@gmail.com> wrote: > 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 <jbel...@gmail.com> wrote: >> >> What happens if you disable row cache? >> >> On Tue, May 25, 2010 at 4:53 AM, Ran Tavory <ran...@gmail.com> wrote: >> > 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 >> http://riptano.com > > -- Jonathan Ellis Project Chair, Apache Cassandra co-founder of Riptano, the source for professional Cassandra support http://riptano.com
