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From "Corentin Chary (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13418) Allow TWCS to ignore overlaps when dropping fully expired sstables
Date Sun, 09 Apr 2017 18:20:42 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15962212#comment-15962212
] 

Corentin Chary commented on CASSANDRA-13418:
--------------------------------------------

If I understand things correctly here, the worst that can happen is that data could re-appear.
Remember that we just drop SSTables were *all* the items have expired.

(The worst that can happen if you don't have the option is that you suddently stop dropping
SSTables and all your disk full up.)

> 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
>
> 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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