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Date: Mon, 20 Mar 2017 15:13:41 +0000 (UTC)
From: "Jan Karlsson (JIRA)" <jira@apache.org>
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Subject: [jira] [Created] (CASSANDRA-13354) LCS estimated compaction tasks
 does not take number of files into account
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archived-at: Mon, 20 Mar 2017 15:13:46 -0000

Jan Karlsson created CASSANDRA-13354:
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             Summary: 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


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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