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Write performance actually decreases as you add ZK servers, while read performance increases modestly: http://bit.ly/9JEUjuzookeeper.apache.org/doc/current/zookeeperOver.html#Performance
See this page for a survey Patrick Hunt (http://twitter.com/phunt) did looking at operational latency with both standalone server and an ensemble of size 3. You'll notice that a single core machine running a standalone ZK ensemble (1 server) is still able to process 15k requests per second. This is orders of magnitude greater than what most applications require (if they are using ZooKeeper correctly - ie as a coordination service, and not as a replacement for a database, filestore, cache, etc...)
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