Andrew Whang created CASSANDRA-13561:
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Summary: Purge TTL on expiration
Key: CASSANDRA-13561
URL: https://issues.apache.org/jira/browse/CASSANDRA-13561
Project: Cassandra
Issue Type: New Feature
Reporter: Andrew Whang
Priority: Minor
Tables with mostly TTL columns tend to suffer from high droppable tombstone
ratio, which results in higher read latency, cpu utilization, and disk usage.
Expired TTL data become tombstones, and the nature of purging tombstones during
compaction (due to checking for overlapping SSTables) make them susceptible to
surviving much longer than expected. A table option to purge TTL on expiration
would address this issue, by preventing them from becoming tombstones. A
boolean purge_ttl_on_expiration table setting would allow users to easily turn
the feature on or off.
Being more aggressive with gc_grace could also address the problem of long
lasting tombstones, but that would affect tombstones from deletes as well.
Even if a purged [expired] cell is revived via repair from a node that hasn't
yet compacted away the cell, it would be revived as an expiring cell with the
same localDeletionTime, so reads should properly handle them. As well, it would
be purged in the next compaction.
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