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From Erik Forsberg <>
Subject On Bloom filters and Key Cache
Date Wed, 21 Mar 2012 15:27:48 GMT

We're currently testing Cassandra with a large number of row keys per 
node - nodetool cfstats approximated number of keys to something like 
700M per node. This seems to have caused a very large heap consumption.

After reading I think I've 
tracked this down to the bloom filter, and the sampled index entries.

Regarding bloom filters, have I understood correctly that they are 
stored on Heap, and that the "Bloom Filter Space Used" reported by 
'nodetool cfstats' is an approximation of the heap space used by bloom 
filters? It reports the on-disk size, but if I understand 
CASSANDRA-3497, the on-disk size is smaller than the on-Heap size?

I understand that increasing bloom_filter_fp_chance will decrease the 
bloom filter size, but at the cost of worse performance when asking for 
keys that don't exist. I do have a fair amount of queries for keys that 
don't exist.

How much will increasing the key cache help, i.e. decrease bloom filter 
size but increase key cache size? Will the key cache cache negative 
results, i.e. the fact that a key didn't exist?


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