Sam Bisbee created CASSANDRA-11060:
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Summary: Allow DTCS old SSTable filtering to use min timestamp
instead of max
Key: CASSANDRA-11060
URL: https://issues.apache.org/jira/browse/CASSANDRA-11060
Project: Cassandra
Issue Type: Improvement
Reporter: Sam Bisbee
We have observed a DTCS behavior when using TTLs where SSTables are never or
very rarely fully expired due to compaction, allowing expired data to be
"stuck" in large partially expired SSTables.
This is because compaction filtering is performed on the max timestamp, which
continues to grow as SSTables are compacted together. This means they will
never move past max_sstable_age_days. With a sufficiently large TTL, like 30
days, this allows old but not expired SSTables to continue combining and never
become fully expired, even with a max_sstable_age_days of 1.
As a result we have seen expired data hang around in large SSTables for over
six months longer than it should have. This is obviously wasteful and a disk
capacity issue.
As a result we have been running an extended version of DTCS called MTCS in
some deployments. The only change is that it uses min timestamp instead of max
for compaction filtering (filterOldSSTables()). This allows SSTables to move
beyond max_sstable_age_days and stop compacting, which means the entire SSTable
can become fully expired and be dropped off disk as intended.
You can see and test MTCS here: https://github.com/threatstack/mtcs
I am not advocating that MTCS be its own stand alone compaction strategy.
However, I would like to see a configuration option for DTCS that allows you to
specify whether old SSTables should be filtered on min or max timestamp.
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