[jira] [Created] (CASSANDRA-15752) Range read concurrency factor didn't consider range merger

["ZhaoYang (Jira)" <ji...@apache.org>]

ZhaoYang created CASSANDRA-15752:
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             Summary: Range read concurrency factor didn't consider range merger
                 Key: CASSANDRA-15752
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-15752
             Project: Cassandra
          Issue Type: Bug
          Components: Legacy/Coordination
            Reporter: ZhaoYang


During range read, coordinator computes concurrency factor which is the number of vnode ranges to contact in parallel for the next batch.

But in {{RangeCommandIterator}}, vnode ranges are merged by {{RangeMerger}} if vnode ranges share enough replicas to satisfy consistency level. eg. vnode range [a,b) has replica n1,n2,n3 and vnode range [b,c) has replica n2,n3,n4, so they can be merged as range [a,c) with replica n2, n3 for Quorum.

Currently it counts number of merged ranges towards concurrency factor. Coordinator may fetch more ranges than needed.

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Another issue is that when executing range read on table with very small amount of data, concurrency factor can be bumped to {{size of total vnode ranges}}, eg. 10k, depending on the num of vnodes and cluster size. As a result, coordinator will send large number of concurrent range requests, potentially slowing down the cluster.. We should cap the max concurrency factor..



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