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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8459) "autocompaction" on reads can prevent memtable space reclaimation
Date Thu, 11 Dec 2014 16:10:15 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14242738#comment-14242738
] 

Benedict commented on CASSANDRA-8459:
-------------------------------------

It's probably not a *bad idea* for 2.0 as it stops a read touching the write path, but it
isn't necessary for correctness.

> "autocompaction" on reads can prevent memtable space reclaimation
> -----------------------------------------------------------------
>
>                 Key: CASSANDRA-8459
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8459
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>             Fix For: 2.1.3
>
>         Attachments: 8459.txt
>
>
> Memtable memory reclamation is dependent on reads always making progress, however on
the collectTimeOrderedData critical path it is possible for the read to perform a _write_
inline, and for this write to block waiting for memtable space to be reclaimed. However the
reclaimation is blocked waiting for this read to complete.
> There are a number of solutions to this, but the simplest is to make the defragmentation
happen asynchronously, so the read terminates normally.



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