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From "Marcus Eriksson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7019) Improve tombstone compactions
Date Wed, 13 Jan 2016 13:40:40 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7019?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15096196#comment-15096196
] 

Marcus Eriksson commented on CASSANDRA-7019:
--------------------------------------------

Perhaps we could apply it to the higher (L3+?) levels in leveled compaction? Or perhaps the
highest and highest - 1 levels. I see two reasons for this;

* we would reduce the number of sstables we have to read when doing these compactions - since
most sstables in L0 overlap all other sstables we would read everything on disk if we did
this for all levels. An sstable in L3 will only overlap 1 (or 2) sstables in each of L2 and
L1. 
* data in the higher levels should be older, meaning we most often can drop more covered data/tombstones
when doing the high level compactions

Question: (I only quickly skimmed the code) do we drop overwritten data as well?

This approach will not be 'perfect' like the 'compact all sstables 1:1'-approach outlined
above, but I like it - it is quite simple to reason about and should give good results (if
the performance does not turn out to be horrible).

> Improve tombstone compactions
> -----------------------------
>
>                 Key: CASSANDRA-7019
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7019
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Compaction
>            Reporter: Marcus Eriksson
>            Assignee: Branimir Lambov
>              Labels: compaction
>             Fix For: 3.x
>
>
> When there are no other compactions to do, we trigger a single-sstable compaction if
there is more than X% droppable tombstones in the sstable.
> In this ticket we should try to include overlapping sstables in those compactions to
be able to actually drop the tombstones. Might only be doable with LCS (with STCS we would
probably end up including all sstables)



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