title: Why some of HDFS reads are so slow, when all the data is cached in off-heap?
body: <p>I'm doing performance test for HDFS. I'm using HDFS C++ API. I notice that most of the reads take up to 5 microseconds. But some of reads take 500-800 microseconds. What could be the reason, considering all the data is in OS cache? No disk access should be occur.</p>