[jira] [Created] (CASSANDRA-21041) SSTable overlap statistics

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

Branimir Lambov created CASSANDRA-21041:
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             Summary: SSTable overlap statistics
                 Key: CASSANDRA-21041
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-21041
             Project: Apache Cassandra
          Issue Type: Improvement
          Components: Local/Compaction, Local/Compaction/UCS
            Reporter: Branimir Lambov


When trying to evaluate the effect of compaction and its settings, perhaps the most relevant information is the amount of overlap between sstables, i.e. the number of sstables that have to be queried to serve any request for a specific partition. We do not currently present this information for any compaction strategy.

For some strategies this can be organized by level, and specifically for UCS, by shard.

[This patch in the DataStax repository|https://github.com/datastax/cassandra/commit/c7d6f082e9eef8f9121ef271495e9a2469bde671] implements such a statistic for UCS, which outputs overlap per level as
{code}
Max overlap map for density.blobs:
  all: 26, per shard: [11, 11, 17, 17, 14, 14, 20, 20, 17, 17, 23, 23, 20, 20, 26, 26]
  unrepaired:0: 18, per shard: [6, 10, 14, 18]
  unrepaired:1: 9, per shard: [6, 9, 5, 8, 4, 7, 3, 6]
  unrepaired:2: 3, per shard: [3, 3, 3, 3, 3, 3, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2, 3, 3, 3, 3, 3, 3, 2, 2]
{code}.



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