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From Peter Schüller <>
Subject Re: boonfilters
Date Wed, 07 Apr 2010 20:34:29 GMT
(bloomfilters, not boonfilters)

Speaking in general, not specific to cassandra:

> 2. Are boonfilters a fixed size, or they adjust as to the # of keys?  any
> example size?

Bloom filters are by their very nature lossy in the sense that you
cannot determine later what you put into it. Re-sizing a bloom filter
implies re-creating it from scratch. I'm not sure what cassandra does

> 3. Boonfilters don't give false negatives:
>    So you hit a node, and perform a lookup in the boonfilter for a key.  It
> says "yes", but when you do a lookup the object returned is null, so then
> you flag that this node needs this particular key during replication.

Someone correct me if I'm wrong, but my understanding is that the
bloom filters are only used to optimize the case of a key being
non-existent, such that you only have to go down on disk for a very
small number of requests for non-existent keys. I do not believe bloom
filters have any interaction with replication. The reason to use a
bloom filter is that it trades great memory efficiency for the
possibility of false positives.

/ Peter Schuller aka scode

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