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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 Wed, 26 Apr 2017 11:18:04 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15984600#comment-15984600
] 

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

[~krummas]: good point, CASSANDRA-10496 seems to come with its own set of issues: number of
sstables would probably get huge, except if you add some kind of "buffering" like what is
done for the first window.
I'll see if I can find a reasonable solution for TWCS or propose it in the related ticket.
If can't agree on a good solution, we can fallback to what is proposed here.

[~adejanovski]: about skipping getOverlappingSSTables() completely, I though about that too,
but I think it's used in some other place and I wasn't sure what the result would be.

> 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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