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From "Jeremiah Jordan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7139) Default concurrent_compactors is probably too high
Date Tue, 17 Jun 2014 17:46:14 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14034093#comment-14034093
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Jeremiah Jordan commented on CASSANDRA-7139:
--------------------------------------------

Can we get this change in 2.0?  Have had the default concurrent compactors causes issues on
a few clusters.

> Default concurrent_compactors is probably too high
> --------------------------------------------------
>
>                 Key: CASSANDRA-7139
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7139
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Jonathan Ellis
>            Priority: Minor
>             Fix For: 2.1 rc1
>
>         Attachments: 7139.txt
>
>
> The default number of concurrent compactors is probably too high for modern hardware
with spinning disks for storage: A modern blade can easily have 24+ Cores, which would result
in a default of 24 concurrent compactions. This not only increases random IO, it also keeps
around a lot of obsoleted files for an unnecessarily long time, as each compaction keeps references
to any possibly overlapping files that it isn't itself compacting - but these can have been
obsoleted part way through by compactions that finished earlier. If you factor in the default
compaction throughput rate of 16Mb/s, anything but a single default concurrent_compactor makes
very little sense, as a single thread should always be able to handle 16Mb/s, will cause less
interference with other processes, and permits obsoleted files to be immediately removed.
> See [http://imgur.com/HDqhxFp] for a graph demonstrating the result of making this change
on a box with 24-cores and 8Tb of storage (first spike is default settings)



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