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https://issues.apache.org/jira/browse/CASSANDRA-21041?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18040236#comment-18040236
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Branimir Lambov commented on CASSANDRA-21041:
---------------------------------------------
Presented as a graph, the above statistics look like this:
!overlap-graph.png!
The three shaded regions are numbers of sstables overlapping in each shard and
level, which add up to a little more than the total overlap in each shard shown
in the dashed black line. The difference between shards is due to the
compactions still ongoing or yet to be started, including the effect of opening
sstables early. The difference between the dashed line and the sum of the
shaded regions is due to some data in non-intersecting regions of some levels,
in this case due to early open resulting in data in a shard being counted both
for the lower level (parts still to be compacted) and the higher (parts already
compacted and opened early).
> 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
> Priority: Normal
> Attachments: overlap-graph.png
>
>
> 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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