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https://issues.apache.org/jira/browse/CASSANDRA-15752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17139340#comment-17139340
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Andres de la Peña commented on CASSANDRA-15752:
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Running cassandra-ci:
||[utest|https://ci-cassandra.apache.org/view/all/job/Cassandra-devbranch-test/130/]||[dtest|https://ci-cassandra.apache.org/view/all/job/Cassandra-devbranch-dtest/167/]||[jvm-dtest|https://ci-cassandra.apache.org/view/all/job/Cassandra-devbranch-jvm-dtest/106/]||
> 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
> Assignee: ZhaoYang
> Priority: Normal
> Fix For: 3.0.x, 3.11.x, 4.0-beta
>
>
> 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.
> ----
> 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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