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https://issues.apache.org/jira/browse/CASSANDRA-13354?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Jan Karlsson updated CASSANDRA-13354:
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Attachment: patchedTest.png
> LCS estimated compaction tasks does not take number of files into account
> -------------------------------------------------------------------------
>
> Key: CASSANDRA-13354
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13354
> Project: Cassandra
> Issue Type: Bug
> Components: Compaction
> Environment: Cassandra 2.2.9
> Reporter: Jan Karlsson
> Assignee: Jan Karlsson
> Attachments: 13354-trunk.txt, patchedTest.png, unpatchedTest.png
>
>
> In LCS, the way we estimate number of compaction tasks remaining for L0 is by
> taking the size of a SSTable and multiply it by four. This would give 4*160mb
> with default settings. This calculation is used to determine whether repaired
> or repaired data is being compacted.
> Now this works well until you take repair into account. Repair streams over
> many many sstables which could be smaller than the configured SSTable size
> depending on your use case. In our case we are talking about many thousands
> of tiny SSTables. As number of files increases one can run into any number of
> problems, including GC issues, too many open files or plain increase in read
> latency.
> With the current algorithm we will choose repaired or unrepaired depending on
> whichever side has more data in it. Even if the repaired files outnumber the
> unrepaired files by a large margin.
> Similarily, our algorithm that selects compaction candidates takes up to 32
> SSTables at a time in L0, however our estimated task calculation does not
> take this number into account. These two mechanisms should be aligned with
> each other.
> I propose that we take the number of files in L0 into account when estimating
> remaining tasks.
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