Stefan Podkowinski created CASSANDRA-13024:
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Summary: Droppable Tombstone Ratio Calculation
Key: CASSANDRA-13024
URL: https://issues.apache.org/jira/browse/CASSANDRA-13024
Project: Cassandra
Issue Type: Bug
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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