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https://issues.apache.org/jira/browse/CASSANDRA-13024?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Stefan Podkowinski updated CASSANDRA-13024:
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Component/s: Compaction
> Droppable Tombstone Ratio Calculation
> -------------------------------------
>
> Key: CASSANDRA-13024
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13024
> Project: Cassandra
> Issue Type: Bug
> Components: Compaction
> Reporter: Stefan Podkowinski
>
> Whenever no sstables can be compacted in a standard way, we currently try to
> compact sstables that would make worthwhile candidates by evaluation
> {{sstable.getEstimatedDroppableTombstoneRatio(gcBefore) <=
> tombstoneThreshold}}. You can find out more on how this is supposed to work
> and the various settings options by reading "[About Deletes and Tombstones in
> Cassandra|http://thelastpickle.com/blog/2016/07/27/about-deletes-and-tombstones.html#single-sstable-compaction]";
> by [~arodrime].
> The bad news is that currently the ratio value will vary to a great degree
> depending on the data model and type of deletes, as the ratio will be created
> based on the number of tombstones and number of _columns_. Any kind of
> tombstone will be counted here, no matter if on column or partition level,
> which will give you very different ratios based on the number of columns in
> your table and how you delete the data.
> Considering a 0.20 default threshold for finding sstables with enough
> droppable tombstones, it only takes 3 columns in your table to never hit the
> threshold at all when using partition tombstones.
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