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From "Romain GERARD (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13418) Allow TWCS to ignore overlaps when dropping fully expired sstables
Date Wed, 28 Jun 2017 17:20:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16066894#comment-16066894
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Romain GERARD commented on CASSANDRA-13418:
-------------------------------------------

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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