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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Updated] (CASSANDRA-4341) Small SSTable Segments Can Hurt Leveling Process
Date Thu, 14 Jun 2012 19:29:42 GMT


Jonathan Ellis updated CASSANDRA-4341:

    Attachment: 4341.txt

Patch attached with the following approach:

.           // We treat L0 compactions specially:
            // 1a. add sstables to the candidate set until we have at least maxSSTableSizeInMB
            // 1b. prefer choosing older sstables as candidates, to newer ones
            // 1c. any L0 sstables that overlap a candidate, will also become candidates
            // 2. At most MAX_COMPACTING_L0 sstables will be compacted at once
            // 3. If total candidate size is less than maxSSTableSizeInMB, we won't bother
compacting with L1,
            //    and the result of the compaction will stay in L0 instead of being promoted
(see promote())
> Small SSTable Segments Can Hurt Leveling Process
> ------------------------------------------------
>                 Key: CASSANDRA-4341
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 1.0.0
>            Reporter: Benjamin Coverston
>              Labels: compaction
>             Fix For: 1.1.2
>         Attachments: 4341.txt
> This concerns:
> static int MAX_COMPACTING_L0 = 32;
> Repair can create very small SSTable segments. We should consider moving to a threshold
that takes into account the size of the files brought into compaction rather than the number
of files for this and similar situations. Bringing the small files from L0 to L1 magnifies
the issue.
> If there are too many very small files in L0 perhaps even an intermediate compaction
would even reduce the magnifying effect of a L0 to L1 compaction.

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