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Romain GERARD commented on CASSANDRA-13418:
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Hi back Marcus,
So I took into account your comments and regarding your the 1rst one I wanted
to do that at first but
getFullyExpiredSSTables is also used in CompactionTask
https://github.com/criteo-forks/cassandra/blob/trunk/src/java/org/apache/cassandra/db/compaction/CompactionTask.java#L165
So only modifying things at the TWS level would have resulted in compacting the
sstables that we wanted to drop, and I was not too incline to touch to
CompactionTask.
It is also making worthDroppingTombstones ignoring overlaps
https://github.com/criteo-forks/cassandra/blob/trunk/src/java/org/apache/cassandra/db/compaction/TimeWindowCompactionStrategy.java#L141
and respect the tombstoneThresold specified (We can turn on
uncheckedTombstoneCompaction for this one)
Regarding the 2nd question I put the code validating the options in
TimeWindowCompactionStategyOptions
https://github.com/criteo-forks/cassandra/blob/cassandra-3.11-criteo/src/java/org/apache/cassandra/db/compaction/TimeWindowCompactionStrategyOptions.java#L157
in order to trigger an exception if the options is used elsewhere than TWCS.
https://github.com/criteo-forks/cassandra/blob/cassandra-3.11-criteo/src/java/org/apache/cassandra/schema/CompactionParams.java#L161
P.s: I will have more time in the upcoming days, so I will be more responsive.
> Allow TWCS to ignore overlaps when dropping fully expired sstables
> ------------------------------------------------------------------
>
> Key: CASSANDRA-13418
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13418
> Project: Cassandra
> Issue Type: Improvement
> Components: Compaction
> Reporter: Corentin Chary
> Labels: twcs
> Attachments: twcs-cleanup.png
>
>
> http://thelastpickle.com/blog/2016/12/08/TWCS-part1.html explains it well. If
> you really want read-repairs you're going to have sstables blocking the
> expiration of other fully expired SSTables because they overlap.
> You can set unchecked_tombstone_compaction = true or tombstone_threshold to a
> very low value and that will purge the blockers of old data that should
> already have expired, thus removing the overlaps and allowing the other
> SSTables to expire.
> The thing is that this is rather CPU intensive and not optimal. If you have
> time series, you might not care if all your data doesn't exactly expire at
> the right time, or if data re-appears for some time, as long as it gets
> deleted as soon as it can. And in this situation I believe it would be really
> beneficial to allow users to simply ignore overlapping SSTables when looking
> for fully expired ones.
> To the question: why would you need read-repairs ?
> - Full repairs basically take longer than the TTL of the data on my dataset,
> so this isn't really effective.
> - Even with a 10% chances of doing a repair, we found out that this would be
> enough to greatly reduce entropy of the most used data (and if you have
> timeseries, you're likely to have a dashboard doing the same important
> queries over and over again).
> - LOCAL_QUORUM is too expensive (need >3 replicas), QUORUM is too slow.
> I'll try to come up with a patch demonstrating how this would work, try it on
> our system and report the effects.
> cc: [~adejanovski], [~rgerard] as I know you worked on similar issues already.
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