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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6357) Flush memtables to separate directory
Date Tue, 06 May 2014 21:36:17 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6357?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13991165#comment-13991165
] 

Benedict commented on CASSANDRA-6357:
-------------------------------------

Unfortunately I can't get the revert patch to apply, but it looks to me that either it's missing
some reverts of method names in Directories or calls to getDirectoryForCompactedSSTables()
 should not have been reverted to getDirectoryForNewSSTables() 

> Flush memtables to separate directory
> -------------------------------------
>
>                 Key: CASSANDRA-6357
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6357
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core
>            Reporter: Patrick McFadin
>            Assignee: Jonathan Ellis
>            Priority: Minor
>              Labels: performance
>             Fix For: 2.1 rc1
>
>         Attachments: 6357-revert-v2.txt, 6357-revert.txt, 6357-v2.txt, 6357.txt, c6357-2.1-stress-write-adj-ops-sec.png,
c6357-2.1-stress-write-latency-99th.png, c6357-2.1-stress-write-latency-median.png, c6357-stress-write-latency-99th-1.png
>
>
> Flush writers are a critical element for keeping a node healthy. When several compactions
run on systems with low performing data directories, IO becomes a premium. Once the disk subsystem
is saturated, write IO is blocked which will cause flush writer threads to backup. Since memtables
are large blocks of memory in the JVM, too much blocking can cause excessive GC over time
degrading performance. In the worst case causing an OOM.
> Since compaction is running on the data directories. My proposal is to create a separate
directory for flushing memtables. Potentially we can use the same methodology of keeping the
commit log separate and minimize disk contention against the critical function of the flushwriter.




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